diff --git a/src/proxy/middle_relay.rs b/src/proxy/middle_relay.rs
index 01ccc20..09dd532 100644
--- a/src/proxy/middle_relay.rs
+++ b/src/proxy/middle_relay.rs
@@ -263,7 +263,14 @@ where
         }
     }
 
-    client_writer.flush().await.map_err(ProxyError::Io)
+    // Avoid unconditional per-frame flush (throughput killer on large downloads).
+    // Flush only when low-latency ack semantics are requested or when
+    // CryptoWriter has buffered pending ciphertext that must be drained.
+    if quickack || client_writer.has_pending() {
+        client_writer.flush().await.map_err(ProxyError::Io)?;
+    }
+
+    Ok(())
 }
 
 async fn write_client_ack<W>(
@@ -283,5 +290,6 @@ where
         .write_all(&bytes)
         .await
         .map_err(ProxyError::Io)?;
+    // ACK should remain low-latency.
     client_writer.flush().await.map_err(ProxyError::Io)
 }
diff --git a/src/transport/middle_proxy/registry.rs b/src/transport/middle_proxy/registry.rs
index abb2b0c..a6a81c8 100644
--- a/src/transport/middle_proxy/registry.rs
+++ b/src/transport/middle_proxy/registry.rs
@@ -9,7 +9,7 @@ use std::sync::Arc;
 use tokio::sync::Mutex;
 
 pub struct ConnRegistry {
-    map: RwLock<HashMap<u64, mpsc::Sender<MeResponse>>>,
+    map: RwLock<HashMap<u64, mpsc::UnboundedSender<MeResponse>>>,
     writers: RwLock<HashMap<u64, Arc<Mutex<RpcWriter>>>>,
     next_id: AtomicU64,
 }
@@ -25,9 +25,11 @@ impl ConnRegistry {
         }
     }
 
-    pub async fn register(&self) -> (u64, mpsc::Receiver<MeResponse>) {
+    pub async fn register(&self) -> (u64, mpsc::UnboundedReceiver<MeResponse>) {
         let id = self.next_id.fetch_add(1, Ordering::Relaxed);
-        let (tx, rx) = mpsc::channel(256);
+        // Unbounded per-connection queue prevents reader-loop HOL blocking on
+        // slow clients: routing stays non-blocking and preserves message order.
+        let (tx, rx) = mpsc::unbounded_channel();
         self.map.write().await.insert(id, tx);
         (id, rx)
     }
@@ -40,7 +42,7 @@ impl ConnRegistry {
     pub async fn route(&self, id: u64, resp: MeResponse) -> bool {
         let m = self.map.read().await;
         if let Some(tx) = m.get(&id) {
-            tx.send(resp).await.is_ok()
+            tx.send(resp).is_ok()
         } else {
             false
         }