diff --git a/data/hlint.yaml b/data/hlint.yaml
index f007f2b2..26fc85e5 100644
--- a/data/hlint.yaml
+++ b/data/hlint.yaml
@@ -1476,6 +1476,46 @@
     - warn: {lhs: foldr' f c (reverse x), rhs: foldl' (flip f) c x, name: Use left fold instead of right fold}
     - warn: {lhs: foldl' f c (reverse x), rhs: foldr (flip f) c x, note: IncreasesLaziness, name: Use right fold instead of left fold}
 
+
+- group:
+    name: performance
+    enabled: false
+    rules:
+
+    # Union with empty is the identity. These show up in generic code paths
+    # ("combine the default with the extras") where the extras are often empty,
+    # and in accumulator-style folds that always start from an empty map.
+    # `union` is O(m + n); dropping the empty side saves the traversal.
+    - warn: {lhs: Data.Map.Strict.union m Data.Map.Strict.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.Map.Strict.union Data.Map.Strict.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.Map.Lazy.union m Data.Map.Lazy.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.Map.Lazy.union Data.Map.Lazy.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.IntMap.Strict.union m Data.IntMap.Strict.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.IntMap.Strict.union Data.IntMap.Strict.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.IntMap.Lazy.union m Data.IntMap.Lazy.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.IntMap.Lazy.union Data.IntMap.Lazy.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashMap.Strict.union m Data.HashMap.Strict.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashMap.Strict.union Data.HashMap.Strict.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashMap.Lazy.union m Data.HashMap.Lazy.empty, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashMap.Lazy.union Data.HashMap.Lazy.empty m, rhs: m, name: "Redundant union with empty"}
+    - warn: {lhs: Data.Set.union s Data.Set.empty, rhs: s, name: "Redundant union with empty"}
+    - warn: {lhs: Data.Set.union Data.Set.empty s, rhs: s, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashSet.union s Data.HashSet.empty, rhs: s, name: "Redundant union with empty"}
+    - warn: {lhs: Data.HashSet.union Data.HashSet.empty s, rhs: s, name: "Redundant union with empty"}
+
+    # `map id` over a container is a full structural rebuild that produces the
+    # same container. Dropping it saves O(n) allocation. Safe on every
+    # variant: strict containers force values to WHNF on insertion by
+    # contract, so they never hold bottoms, and `map id` on a lazy container
+    # is a pure identity.
+    - warn: {lhs: Data.Map.Strict.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.Map.Lazy.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.IntMap.Strict.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.IntMap.Lazy.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.HashMap.Strict.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.HashMap.Lazy.map id m, rhs: m, name: "Redundant map id"}
+    - warn: {lhs: Data.Set.map id s, rhs: s, name: "Redundant map id"}
+    - warn: {lhs: Data.HashSet.map id s, rhs: s, name: "Redundant map id"}
 - group:
     # used for tests, enabled when testing this file
     name: testing
diff --git a/tests/flag-with-group-performance-containers.test b/tests/flag-with-group-performance-containers.test
new file mode 100644
index 00000000..a91da319
--- /dev/null
+++ b/tests/flag-with-group-performance-containers.test
@@ -0,0 +1,133 @@
+---------------------------------------------------------------------
+FILE tests/flag-with-group-performance-containers.hs
+{-# HLINT ignore "Avoid restricted alias" #-}
+import qualified Data.Map.Strict as MS
+import qualified Data.Map.Lazy as ML
+import qualified Data.IntMap.Strict as IMS
+import qualified Data.IntMap.Lazy as IML
+import qualified Data.HashMap.Strict as HMS
+import qualified Data.HashMap.Lazy as HML
+import qualified Data.Set as Set
+import qualified Data.HashSet as HSet
+uMSr m = print (MS.union m MS.empty)
+uMSl m = print (MS.union MS.empty m)
+uMLr m = print (ML.union m ML.empty)
+uIMSr m = print (IMS.union m IMS.empty)
+uIMLr m = print (IML.union m IML.empty)
+uHMSr m = print (HMS.union m HMS.empty)
+uHMLr m = print (HML.union m HML.empty)
+uSr s = print (Set.union s Set.empty)
+uHSr s = print (HSet.union s HSet.empty)
+miMS m = print (MS.map id m)
+miML m = print (ML.map id m)
+miIMS m = print (IMS.map id m)
+miIML m = print (IML.map id m)
+miHMS m = print (HMS.map id m)
+miHML m = print (HML.map id m)
+miS s = print (Set.map id s)
+miHS s = print (HSet.map id s)
+RUN "--with-group=performance" tests/flag-with-group-performance-containers.hs
+OUTPUT
+tests/flag-with-group-performance-containers.hs:10:17-35: Warning: Redundant union with empty
+Found:
+  MS.union m MS.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:11:17-35: Warning: Redundant union with empty
+Found:
+  MS.union MS.empty m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:12:17-35: Warning: Redundant union with empty
+Found:
+  ML.union m ML.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:13:18-38: Warning: Redundant union with empty
+Found:
+  IMS.union m IMS.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:14:18-38: Warning: Redundant union with empty
+Found:
+  IML.union m IML.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:15:18-38: Warning: Redundant union with empty
+Found:
+  HMS.union m HMS.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:16:18-38: Warning: Redundant union with empty
+Found:
+  HML.union m HML.empty
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:17:16-36: Warning: Redundant union with empty
+Found:
+  Set.union s Set.empty
+Perhaps:
+  s
+
+tests/flag-with-group-performance-containers.hs:18:17-39: Warning: Redundant union with empty
+Found:
+  HSet.union s HSet.empty
+Perhaps:
+  s
+
+tests/flag-with-group-performance-containers.hs:19:17-27: Warning: Redundant map id
+Found:
+  MS.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:20:17-27: Warning: Redundant map id
+Found:
+  ML.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:21:18-29: Warning: Redundant map id
+Found:
+  IMS.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:22:18-29: Warning: Redundant map id
+Found:
+  IML.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:23:18-29: Warning: Redundant map id
+Found:
+  HMS.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:24:18-29: Warning: Redundant map id
+Found:
+  HML.map id m
+Perhaps:
+  m
+
+tests/flag-with-group-performance-containers.hs:25:16-27: Warning: Redundant map id
+Found:
+  Set.map id s
+Perhaps:
+  s
+
+tests/flag-with-group-performance-containers.hs:26:17-29: Warning: Redundant map id
+Found:
+  HSet.map id s
+Perhaps:
+  s
+
+17 hints