Add todo for performance improvement

src/cache.rs

@@ -251,6 +251,7 @@ impl SyncCacheCommandSender {
     pub fn send_cache_commands(&self, pages: &[PageNumber], height: i32) {
         for &page in pages {
             // Make message in front the most important
+            // TODO: the low res cach will never be actually done
             self.channel
                 .borrow_mut()
                 .cache_commands