diff --git a/crates/matrix-sdk/src/sliding_sync/mod.rs b/crates/matrix-sdk/src/sliding_sync/mod.rs
index df2c83612..73fab5830 100644
--- a/crates/matrix-sdk/src/sliding_sync/mod.rs
+++ b/crates/matrix-sdk/src/sliding_sync/mod.rs
@@ -354,9 +354,11 @@ impl SlidingSync {
         }
 
         let update_summary = {
-            let mut updated_rooms = Vec::new();
             let mut rooms_map = self.inner.rooms.write().unwrap();
 
+            // Update the rooms.
+            let mut updated_rooms = Vec::with_capacity(sliding_sync_response.rooms.len());
+
             for (room_id, mut room_data) in sliding_sync_response.rooms.into_iter() {
                 // `sync_response` contains the rooms with decrypted events if any, so look at
                 // the timeline events here first if the room exists.
@@ -390,7 +392,8 @@ impl SlidingSync {
                 updated_rooms.push(room_id);
             }
 
-            let mut updated_lists = Vec::new();
+            // Update the lists.
+            let mut updated_lists = Vec::with_capacity(sliding_sync_response.lists.len());
 
             for (name, updates) in sliding_sync_response.lists {
                 let Some(list) = lists.get_mut(&name) else {