diff --git a/src/client/device_registry.rs b/src/client/device_registry.rs
index ae9fec69b..47d3129b1 100644
--- a/src/client/device_registry.rs
+++ b/src/client/device_registry.rs
@@ -15,13 +15,19 @@ use super::Client;
 enum UserLookupKeys {
     /// User is a LID with known phone number mapping.
     /// Keys: [LID, PN]
-    LidWithPn { lid: String, pn: String },
+    LidWithPn {
+        lid: wacore_binary::CompactString,
+        pn: wacore_binary::CompactString,
+    },
     /// User is a phone number with known LID mapping.
     /// Keys: [LID, PN]
-    PnWithLid { lid: String, pn: String },
+    PnWithLid {
+        lid: wacore_binary::CompactString,
+        pn: wacore_binary::CompactString,
+    },
     /// Unknown user - no LID-PN mapping exists.
     /// Could be either a LID or PN, we don't know.
-    Unknown { user: String },
+    Unknown { user: wacore_binary::CompactString },
 }
 
 impl UserLookupKeys {
@@ -61,26 +67,26 @@ impl Client {
     /// - `PnWithLid`: User is a phone number with known LID mapping
     /// - `Unknown`: No LID-PN mapping exists (could be either type)
     async fn resolve_lookup_keys(&self, user: &str) -> UserLookupKeys {
-        // Check if user is a LID (has a phone number mapping)
+        // Check if user is a LID (has a phone number mapping). The `user`-derived
+        // key is built inline via CompactString (LID/PN are short), avoiding a
+        // heap String per member on every group send.
         if let Some(pn) = self.lid_pn_cache.get_phone_number(user).await {
             return UserLookupKeys::LidWithPn {
-                lid: user.to_string(),
-                pn,
+                lid: user.into(),
+                pn: pn.into(),
             };
         }
 
         // Check if user is a PN (has a LID mapping)
         if let Some(lid) = self.lid_pn_cache.get_current_lid(user).await {
             return UserLookupKeys::PnWithLid {
-                lid,
-                pn: user.to_string(),
+                lid: lid.into(),
+                pn: user.into(),
             };
         }
 
         // Unknown user - no mapping exists
-        UserLookupKeys::Unknown {
-            user: user.to_string(),
-        }
+        UserLookupKeys::Unknown { user: user.into() }
     }
 
     /// Get all possible lookup keys for a user (for bidirectional lookup).