diff --git a/src/allow_failure_test.mbt b/src/allow_failure_test.mbt
index c8d9918..3d652be 100644
--- a/src/allow_failure_test.mbt
+++ b/src/allow_failure_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "allow_failure ignored" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(allow_failure=true, fn() { raise Err })
@@ -32,7 +33,8 @@ async test "allow_failure ignored" {
 
 ///|
 async test "allow_failure waited" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
@@ -47,7 +49,8 @@ async test "allow_failure waited" {
 
 ///|
 async test "allow_failure no error" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
diff --git a/src/cancellation_test.mbt b/src/cancellation_test.mbt
index 505bc63..3da84a5 100644
--- a/src/cancellation_test.mbt
+++ b/src/cancellation_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "manual cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   let result = try? @async.with_task_group(fn(root) {
       let task = root.spawn(allow_failure=true, fn() {
@@ -49,7 +50,8 @@ suberror Err derive(Show)
 
 ///|
 async test "error propagation" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   let result = try? @async.with_task_group(fn(root) {
       root.spawn_bg(fn() {
@@ -85,7 +87,8 @@ async test "error propagation" {
 
 ///|
 async test "multiple scope" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -151,7 +154,8 @@ async test "multiple scope" {
 
 ///|
 async test "recursive cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
@@ -191,7 +195,8 @@ async test "recursive cancel" {
 
 ///|
 async test "spawn directly error" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
@@ -218,7 +223,8 @@ async test "spawn directly error" {
 
 ///|
 async test "error in cancellation" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -244,7 +250,8 @@ async test "error in cancellation" {
 
 ///|
 async test "immediately cancelled" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     let task = root.spawn(allow_failure=true, fn() {
diff --git a/src/fs/walk_test.mbt b/src/fs/walk_test.mbt
index e171121..f09517a 100644
--- a/src/fs/walk_test.mbt
+++ b/src/fs/walk_test.mbt
@@ -28,6 +28,7 @@ async test "walk sequential" {
 
 ///|
 async test "walk parallel" {
+  @async.sleep(300)
   let log = StringBuilder::new()
   @fs.walk("test_directory", max_concurrency=2, fn(path, _) {
     log.write_string("handling \{path}\n")
diff --git a/src/group_defer_test.mbt b/src/group_defer_test.mbt
index 8df1d97..9aa8ad5 100644
--- a/src/group_defer_test.mbt
+++ b/src/group_defer_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "group defer basic" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.add_defer(() => log.write_string("first group defer\n"))
@@ -55,7 +56,8 @@ async test "group defer basic" {
 
 ///|
 async test "group defer error" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -98,7 +100,8 @@ async test "group defer error" {
 
 ///|
 async test "group defer cancelled" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
diff --git a/src/no_wait_test.mbt b/src/no_wait_test.mbt
index 26cb3e2..e6f7aea 100644
--- a/src/no_wait_test.mbt
+++ b/src/no_wait_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "no_wait cancelled" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(no_wait=true, fn() {
@@ -42,7 +43,8 @@ async test "no_wait cancelled" {
 
 ///|
 async test "no_wait normal exit" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(no_wait=true, fn() {
diff --git a/src/protect_from_cancel_test.mbt b/src/protect_from_cancel_test.mbt
index 7577719..486705c 100644
--- a/src/protect_from_cancel_test.mbt
+++ b/src/protect_from_cancel_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "protect_from_cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -62,7 +63,8 @@ async test "protect_from_cancel" {
 
 ///|
 async test "protect_from_cancel wait" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -105,7 +107,8 @@ async test "protect_from_cancel wait" {
 
 ///|
 async test "protect_from_cancel with_timeout" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -137,7 +140,8 @@ async test "protect_from_cancel with_timeout" {
 
 ///|
 async test "protect_from_cancel fail" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -169,7 +173,8 @@ async test "protect_from_cancel fail" {
 
 ///|
 async test "protect_from_cancel nested1" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -203,7 +208,8 @@ async test "protect_from_cancel nested1" {
 
 ///|
 async test "protect_from_cancel nested2" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -243,7 +249,8 @@ async test "protect_from_cancel nested2" {
 
 ///|
 async test "protect_from_cancel in cancellation handler" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -276,7 +283,8 @@ async test "protect_from_cancel in cancellation handler" {
 
 ///|
 async test "cancel while scheduled" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -320,7 +328,8 @@ async test "cancel while scheduled" {
 
 ///|
 async test "error in async cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
diff --git a/src/random_delay_test.mbt b/src/rate_limit_for_test.mbt
similarity index 51%
rename from src/random_delay_test.mbt
rename to src/rate_limit_for_test.mbt
index 1c50ee3..1ca2bf9 100644
--- a/src/random_delay_test.mbt
+++ b/src/rate_limit_for_test.mbt
@@ -13,25 +13,4 @@
 // limitations under the License.
 
 ///|
-/// add a random delay before every test,
-/// so that there won't be a lot of timers expiring at the same time,
-/// causing inaccurate result
-async fn random_delay() -> Unit {
-  let rand = match rand_seed.val {
-    Some(r) => r
-    None => {
-      let now = @async.now()
-      let seed = @buffer.new()
-      for _ in 0..<4 {
-        seed.write_int64_le(now)
-      }
-      let rand = @random.Rand::chacha8(seed=seed.to_bytes())
-      rand_seed.val = Some(rand)
-      rand
-    }
-  }
-  @async.sleep(rand.int(limit=100))
-}
-
-///|
-let rand_seed : Ref[@random.Rand?] = @ref.new(None)
+let rate_limit : @semaphore.Semaphore = @semaphore.Semaphore::new(20)
diff --git a/src/retry_test.mbt b/src/retry_test.mbt
index 1e3f9cc..726858e 100644
--- a/src/retry_test.mbt
+++ b/src/retry_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "retry immediate" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   let mut i = 0
   @async.retry(Immediate, fn() {
@@ -37,7 +38,8 @@ async test "retry immediate" {
 
 ///|
 async test "retry fixed" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -72,7 +74,8 @@ async test "retry fixed" {
 
 ///|
 async test "retry exponential" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -116,7 +119,8 @@ async test "retry exponential" {
 
 ///|
 async test "retry cancelled1" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   let mut i = 0
   @async.with_timeout(500, fn() {
@@ -142,7 +146,8 @@ async test "retry cancelled1" {
 
 ///|
 async test "retry cancelled2" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   let mut i = 0
   @async.with_timeout(500, fn() {
@@ -168,7 +173,8 @@ async test "retry cancelled2" {
 
 ///|
 async test "retry fatal_error" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   fn fatal_error(err) {
     not(err is @async.TimeoutError)
diff --git a/src/return_immediately_test.mbt b/src/return_immediately_test.mbt
index 9e9bbaf..b812a78 100644
--- a/src/return_immediately_test.mbt
+++ b/src/return_immediately_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "return_immediately basic" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -33,7 +34,8 @@ async test "return_immediately basic" {
 
 ///|
 async test "return_immediately nested" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -58,7 +60,8 @@ async test "return_immediately nested" {
 
 ///|
 async test "return_immediately error on cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
diff --git a/src/spawn_loop_test.mbt b/src/spawn_loop_test.mbt
index d826d7f..851e446 100644
--- a/src/spawn_loop_test.mbt
+++ b/src/spawn_loop_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "spawn_loop basic" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -43,7 +44,8 @@ async test "spawn_loop basic" {
 
 ///|
 async test "spawn_loop basic-error" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -71,7 +73,8 @@ async test "spawn_loop basic-error" {
 
 ///|
 async test "spawn_loop retry-exponential" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
diff --git a/src/spawn_test.mbt b/src/spawn_test.mbt
index f024caa..11dbb51 100644
--- a/src/spawn_test.mbt
+++ b/src/spawn_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "spawn basic" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -50,7 +51,8 @@ async test "spawn basic" {
 
 ///|
 async test "spawn error1" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   let result = try? @async.with_task_group(fn(root) {
       root.spawn_bg(fn() {
@@ -68,7 +70,8 @@ async test "spawn error1" {
 
 ///|
 async test "spawn error2" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   let result = try? @async.with_task_group(fn(root) {
       root.spawn_bg(fn() {
@@ -86,7 +89,8 @@ async test "spawn error2" {
 
 ///|
 async test "spawn after cancelled 1" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -120,7 +124,8 @@ async test "spawn after cancelled 1" {
 
 ///|
 async test "spawn after cancelled 2" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
diff --git a/src/timer_test.mbt b/src/timer_test.mbt
index 756a1be..ca1415d 100644
--- a/src/timer_test.mbt
+++ b/src/timer_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "basic_timer" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   let result = try? @async.with_task_group(fn(root) {
       root.spawn_bg(fn() {
diff --git a/src/wait_test.mbt b/src/wait_test.mbt
index debc5d9..f08b086 100644
--- a/src/wait_test.mbt
+++ b/src/wait_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "wait basic" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   buf.write_object(
     try? @async.with_task_group(fn(root) {
@@ -46,7 +47,8 @@ async test "wait basic" {
 
 ///|
 async test "wait cancelled" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let buf = StringBuilder::new()
   @async.with_task_group(fn(root) {
     let task = root.spawn(fn() {
@@ -77,7 +79,8 @@ async test "wait cancelled" {
 
 ///|
 async test "try_wait" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     let task = root.spawn(no_wait=true, fn() {
diff --git a/src/with_timeout_test.mbt b/src/with_timeout_test.mbt
index 1e51504..3915e24 100644
--- a/src/with_timeout_test.mbt
+++ b/src/with_timeout_test.mbt
@@ -14,7 +14,8 @@
 
 ///|
 async test "with_timeout normal exit" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -42,7 +43,8 @@ async test "with_timeout normal exit" {
 
 ///|
 async test "with_timeout failure" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -72,7 +74,8 @@ async test "with_timeout failure" {
 
 ///|
 async test "with_timeout timeout" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -108,7 +111,8 @@ async test "with_timeout timeout" {
 
 ///|
 async test "with_timeout nested1" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -152,7 +156,8 @@ async test "with_timeout nested1" {
 
 ///|
 async test "with_timeout nested2" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -195,7 +200,8 @@ async test "with_timeout nested2" {
 
 ///|
 async test "with_timeout error_on_cancel" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -226,7 +232,8 @@ async test "with_timeout error_on_cancel" {
 
 ///|
 async test "with_timeout error-on-timeout" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -258,7 +265,8 @@ async test "with_timeout error-on-timeout" {
 
 ///|
 async test "with_timeout_opt normal exit" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {
@@ -289,7 +297,8 @@ async test "with_timeout_opt normal exit" {
 
 ///|
 async test "with_timeout_opt failure" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   @async.with_task_group(fn(root) {
     root.spawn_bg(fn() {
@@ -319,7 +328,8 @@ async test "with_timeout_opt failure" {
 
 ///|
 async test "with_timeout_opt timeout" {
-  random_delay()
+  rate_limit.acquire()
+  defer rate_limit.release()
   let log = StringBuilder::new()
   log.write_object(
     try? @async.with_task_group(fn(root) {