# UNDF: UNDF-2026-000000831
# UNDF: (pending)
--- a/src/Simplex/Chat/Library/Internal.hs
+++ b/src/Simplex/Chat/Library/Internal.hs
@@ -30,6 +30,7 @@ import Control.Monad.Except
 import Data.Bifunctor (bimap, first)
 import Data.Fixed (Fixed (..))
 import Data.List (find, foldl', mapAccumL, partition)
+import qualified Data.Set as S
 import Data.Map.Strict (Map)
 import qualified Data.Map.Strict as M
 ...
@@ -1064,9 +1064,11 @@ introduceToRemaining vr user gInfo m = do
   (members, introducedGMIds) <-
     withStore' $ \db -> (,) <$> getGroupMembers db vr user gInfo <*> getIntroducedGroupMemberIds db m
-  let recipients = filter (introduceMemP introducedGMIds) members
+  let introducedSet = S.fromList introducedGMIds
+      recipients = filter (introduceMemP introducedSet) members
   introduceMember vr user gInfo m recipients Nothing
   where
-    introduceMemP introducedGMIds mem =
+    introduceMemP introducedSet mem =
       memberCurrent mem
-        && groupMemberId' mem `notElem` introducedGMIds
+        && groupMemberId' mem `S.notMember` introducedSet
         && groupMemberId' mem /= groupMemberId' m