diff --git a/.config/nextest.toml b/.config/nextest.toml
index 070069fff8..bc3f72a109 100644
--- a/.config/nextest.toml
+++ b/.config/nextest.toml
@@ -34,6 +34,14 @@ telemetry-contract = { max-threads = 1 }
 # parallel load the file never appears (#5355). Serialize them; do not drop
 # the tests.
 exec-persistent-service = { max-threads = 1 }
+# Fleet manager lifecycle tests start real `/bin/sh` workers under setsid and
+# wait 10-15 s for the child's first line. Under full macOS CI load that line
+# never ran for three overlapping tests (#6424, #6428 runs), while each
+# passed alone from the same binary in under 1.2 s. Serialize the module; do
+# not lengthen its deadlines or drop tests. The product side of the load
+# (a durable heartbeat and a `ps` sample on every scheduler tick) is now
+# bounded to once a second per worker.
+fleet-manager-lifecycle = { max-threads = 1 }
 
 # First matching test-group override wins. Keep these more-specific
 # integration filters before binary(integration) so they are not stolen
@@ -56,6 +64,10 @@ test-group = 'telemetry-contract'
 filter = 'binary(telemetry_kill_switch_dispatch)'
 test-group = 'telemetry-contract'
 
+[[profile.default.overrides]]
+filter = 'package(codewhale-tui) & kind(lib) & test(/^fleet::manager::tests::/)'
+test-group = 'fleet-manager-lifecycle'
+
 [[profile.default.overrides]]
 filter = 'binary(integration) & test(/^exec_persistent_service::/)'
 test-group = 'exec-persistent-service'
diff --git a/crates/tui/src/fleet/host.rs b/crates/tui/src/fleet/host.rs
index d332ff5aa3..167b13b37a 100644
--- a/crates/tui/src/fleet/host.rs
+++ b/crates/tui/src/fleet/host.rs
@@ -190,8 +190,14 @@ struct LocalWorkerProcess {
     stopped: bool,
     last_exit: Option<ExitStatus>,
     last_memory_mb: Option<u64>,
+    /// When `ps` last sampled this worker. Status polls can run every few
+    /// milliseconds; memory is display data, so one sample a second is ample.
+    last_memory_sample: Option<std::time::Instant>,
 }
 
+/// Minimum spacing between `ps` memory samples for one worker.
+const MEMORY_SAMPLE_INTERVAL: Duration = Duration::from_secs(1);
+
 impl LocalProcessFleetHostAdapter {
     pub fn new(workspace: impl AsRef<Path>) -> Self {
         Self {
@@ -303,6 +309,7 @@ impl LocalProcessFleetHostAdapter {
                 stopped: false,
                 last_exit: None,
                 last_memory_mb: None,
+                last_memory_sample: None,
             },
         );
         Ok(handle)
@@ -356,7 +363,11 @@ impl FleetHostAdapter for LocalProcessFleetHostAdapter {
         match process.child.try_wait() {
             Ok(None) => {
                 let pid = process.child.id();
-                let memory_mb = if process.host_kind == FleetHostKind::LocalProcess {
+                let due = process
+                    .last_memory_sample
+                    .is_none_or(|sampled| sampled.elapsed() >= MEMORY_SAMPLE_INTERVAL);
+                let memory_mb = if process.host_kind == FleetHostKind::LocalProcess && due {
+                    process.last_memory_sample = Some(std::time::Instant::now());
                     sample_process_memory_mb(pid)
                 } else {
                     None
diff --git a/crates/tui/src/fleet/manager.rs b/crates/tui/src/fleet/manager.rs
index 76534ac1ec..6b81ac471f 100644
--- a/crates/tui/src/fleet/manager.rs
+++ b/crates/tui/src/fleet/manager.rs
@@ -608,11 +608,20 @@ impl FleetManager {
             .ok_or_else(|| anyhow!("Fleet run {} does not exist", run_id.0))?;
         let worker_ids = worker_ids_for_run(&run, max_workers);
 
+        // Heartbeats are durable ledger appends (a full-drive flush on
+        // macOS). Timestamps have whole-second resolution and the stale window
+        // is minutes, so a worker already stamped this second needs no second
+        // record; a fast driver tick must not turn into a flush storm.
+        let now = timestamp();
         for task in active_tasks_for_run(&state, run_id) {
             if let Some(worker_id) = task.leased_to.as_deref()
                 && worker_ids.iter().any(|id| id == worker_id)
+                && state
+                    .heartbeats
+                    .get(worker_id)
+                    .is_none_or(|heartbeat| heartbeat.timestamp != now)
             {
-                self.ledger.heartbeat(worker_id, &timestamp(), None, None)?;
+                self.ledger.heartbeat(worker_id, &now, None, None)?;
                 report.heartbeats += 1;
             }
         }