diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0756d48d25..b14be0cbb8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -57,6 +57,7 @@
 * [BUGFIX] Query Frontend: Fix native histogram responses not being handled correctly in `minTime()` sort ordering for split_by_interval merge. #7555
 * [BUGFIX] Distributor: Release the push worker pool goroutines on shutdown by stopping the async executor during the stopping phase when `-distributor.num-push-workers` is set. #7602
 * [BUGFIX] Querier: Fix unbounded resource leak in the bucket-scan blocks finder (used when the bucket index is disabled). Per-tenant metadata fetchers, their Prometheus registries, and on-disk meta caches are now evicted once a tenant is no longer active, instead of being retained for the lifetime of the process. #7573
+* [BUGFIX] Compactor: Fix flake in `TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning` and its partition twin: shrink the 100-tenant mock-bucket fixture, whose quadratic testify-mock matching could starve the first compaction run past the poll budget on loaded CI runners, and align the partition test's run-completion poll timeout with its non-partition sibling. #7617
 
 ## 1.21.0 2026-04-24
 
diff --git a/pkg/compactor/compactor_paritioning_test.go b/pkg/compactor/compactor_paritioning_test.go
index 0002a131b5..4398a1da36 100644
--- a/pkg/compactor/compactor_paritioning_test.go
+++ b/pkg/compactor/compactor_paritioning_test.go
@@ -1142,7 +1142,7 @@ func TestPartitionCompactor_ShouldCompactAllUsersOnShardingEnabledButOnlyOneInst
 
 func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning(t *testing.T) {
 
-	numUsers := 100
+	numUsers := 20
 
 	// Setup user IDs
 	userIDs := make([]string, 0, numUsers)
@@ -1192,6 +1192,7 @@ func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEn
 		cfg.ShardingRing.InstanceAddr = fmt.Sprintf("127.0.0.%d", i)
 		cfg.ShardingRing.WaitStabilityMinDuration = time.Second
 		cfg.ShardingRing.WaitStabilityMaxDuration = 5 * time.Second
+		cfg.ShardingRing.WaitActiveInstanceTimeout = 30 * time.Second // Give the ring ACTIVE wait headroom on slow CI (#7503).
 		cfg.ShardingRing.KVStore.Mock = kvstore
 
 		c, _, tsdbPlanner, l, _ := prepareForPartitioning(t, cfg, bucketClient, nil, nil)
@@ -1214,7 +1215,7 @@ func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEn
 
 	// Wait until a run has been completed on each compactor
 	for _, c := range compactors {
-		cortex_testutil.Poll(t, 60*time.Second, true, func() any {
+		cortex_testutil.Poll(t, 120*time.Second, true, func() any {
 			return prom_testutil.ToFloat64(c.CompactionRunsCompleted) >= 1
 		})
 	}
diff --git a/pkg/compactor/compactor_test.go b/pkg/compactor/compactor_test.go
index d1d4460383..15b6c1adf1 100644
--- a/pkg/compactor/compactor_test.go
+++ b/pkg/compactor/compactor_test.go
@@ -1091,7 +1091,7 @@ func TestCompactor_ShouldCompactAllUsersOnShardingEnabledButOnlyOneInstanceRunni
 func TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning(t *testing.T) {
 	t.Parallel()
 
-	numUsers := 100
+	numUsers := 20
 
 	// Setup user IDs
 	userIDs := make([]string, 0, numUsers)
@@ -1139,6 +1139,7 @@ func TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndM
 		cfg.ShardingRing.InstanceAddr = fmt.Sprintf("127.0.0.%d", i)
 		cfg.ShardingRing.WaitStabilityMinDuration = time.Second
 		cfg.ShardingRing.WaitStabilityMaxDuration = 5 * time.Second
+		cfg.ShardingRing.WaitActiveInstanceTimeout = 30 * time.Second // Give the ring ACTIVE wait headroom on slow CI (#7503).
 		cfg.ShardingRing.KVStore.Mock = kvstore
 
 		c, _, tsdbPlanner, l, _ := prepare(t, cfg, bucketClient, nil)