diff --git a/src/core/config/mod.rs b/src/core/config/mod.rs
index 881decfa..8ef580f1 100644
--- a/src/core/config/mod.rs
+++ b/src/core/config/mod.rs
@@ -1839,9 +1839,9 @@ pub struct Config {
 	pub stream_amplification: usize,
 
 	/// Number of sender task workers; determines sender parallelism. Default is
-	/// '4'. Override by setting a different value. Values clamped 1 to core count.
+	/// core count. Override by setting a different value.
 	///
-	/// default: 4
+	/// default: core count
 	#[serde(default = "default_sender_workers")]
 	pub sender_workers: usize,
 
@@ -2075,40 +2075,41 @@ fn default_db_write_buffer_capacity_mb() -> f64 { 48.0 + parallelism_scaled_f64(
 
 fn default_db_cache_capacity_mb() -> f64 { 512.0 + parallelism_scaled_f64(512.0) }
 
-fn default_pdu_cache_capacity() -> u32 { parallelism_scaled_u32(50_000).saturating_add(500_000) }
+fn default_pdu_cache_capacity() -> u32 { parallelism_scaled_u32(50_000).saturating_add(100_000) }
 
 fn default_cache_capacity_modifier() -> f64 { 1.0 }
 
 fn default_auth_chain_cache_capacity() -> u32 {
-	parallelism_scaled_u32(50_000).saturating_add(500_000)
+	parallelism_scaled_u32(50_000).saturating_add(100_000)
 }
 
 fn default_shorteventid_cache_capacity() -> u32 {
-	parallelism_scaled_u32(100_000).saturating_add(500_000)
+	parallelism_scaled_u32(100_000).saturating_add(100_000)
 }
 
 fn default_eventidshort_cache_capacity() -> u32 {
-	parallelism_scaled_u32(100_000).saturating_add(500_000)
+	parallelism_scaled_u32(50_000).saturating_add(100_000)
 }
 
 fn default_eventid_pdu_cache_capacity() -> u32 {
-	parallelism_scaled_u32(50_000).saturating_add(500_000)
+	parallelism_scaled_u32(50_000).saturating_add(100_000)
 }
 
 fn default_shortstatekey_cache_capacity() -> u32 {
-	parallelism_scaled_u32(50_000).saturating_add(500_000)
+	parallelism_scaled_u32(50_000).saturating_add(100_000)
 }
 
 fn default_statekeyshort_cache_capacity() -> u32 {
-	parallelism_scaled_u32(50_000).saturating_add(500_000)
+	parallelism_scaled_u32(50_000).saturating_add(100_000)
 }
 
 fn default_servernameevent_data_cache_capacity() -> u32 {
-	parallelism_scaled_u32(200_000).saturating_add(500_000)
+	parallelism_scaled_u32(100_000).saturating_add(100_000)
 }
 
 fn default_stateinfo_cache_capacity() -> u32 {
-	parallelism_scaled_u32(500).clamp(100, 12000) }
+	parallelism_scaled_u32(500).clamp(100, 12000)
+}
 
 fn default_roomid_spacehierarchy_cache_capacity() -> u32 {
 	parallelism_scaled_u32(500).clamp(100, 12000) }
@@ -2328,7 +2329,7 @@ fn default_stream_width_scale() -> f32 { 1.0 }
 
 fn default_stream_amplification() -> usize { 1024 }
 
-fn default_sender_workers() -> usize { 4 }
+fn default_sender_workers() -> usize { parallelism_scaled(1) }
 
 fn default_client_receive_timeout() -> u64 { 75 }
 
diff --git a/src/service/rooms/event_handler/upgrade_outlier_pdu.rs b/src/service/rooms/event_handler/upgrade_outlier_pdu.rs
index 4093cb05..f84c21ce 100644
--- a/src/service/rooms/event_handler/upgrade_outlier_pdu.rs
+++ b/src/service/rooms/event_handler/upgrade_outlier_pdu.rs
@@ -149,7 +149,7 @@ where
 	let extremities: Vec<_> = self
 		.services
 		.state
-		.get_forward_extremities(room_id)
+		.get_forward_extremities(room_id, &state_lock)
 		.map(ToOwned::to_owned)
 		.ready_filter(|event_id| {
 			// Remove any that are referenced by this incoming event's prev_events
@@ -167,6 +167,8 @@ where
 		.collect()
 		.await;
 
+	if extremities.len() == 0 { info!("Retained zero extremities when upgrading outlier PDU to timeline PDU with {} previous events, event id: {}", incoming_pdu.prev_events.len(), incoming_pdu.event_id) }
+	
 	debug!(
 		"Retained {} extremities checked against {} prev_events",
 		extremities.len(),
diff --git a/src/service/rooms/state/mod.rs b/src/service/rooms/state/mod.rs
index 641aa6a9..92881126 100644
--- a/src/service/rooms/state/mod.rs
+++ b/src/service/rooms/state/mod.rs
@@ -388,6 +388,7 @@ impl Service {
 	pub fn get_forward_extremities<'a>(
 		&'a self,
 		room_id: &'a RoomId,
+		_state_lock: &'a RoomMutexGuard,
 	) -> impl Stream<Item = &EventId> + Send + '_ {
 		let prefix = (room_id, Interfix);