diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..5af5241
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+*.swp
+cabal.project.local*
+dist-newstyle
diff --git a/bench/Cuckoo.hs b/bench/Cuckoo.hs
new file mode 100644
index 0000000..e1e81cd
--- /dev/null
+++ b/bench/Cuckoo.hs
@@ -0,0 +1,83 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
+
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
+-- |
+-- Module: Main
+-- Copyright: Copyright © 2019-2021 Lars Kuhtz <lakuhtz@gmail.com>
+-- License: BSD3
+-- Maintainer: Lars Kuhtz <lakuhtz@gmail.com>
+-- Stability: experimental
+--
+module Main
+( main
+) where
+
+import Control.Monad (filterM)
+import Control.Monad.Primitive (PrimState)
+import Control.Monad.ST (runST)
+import Criterion
+import Criterion.Main
+import Data.Foldable (traverse_)
+
+import Data.Cuckoo
+
+-- -------------------------------------------------------------------------- --
+-- Main
+
+main :: IO ()
+main = do
+    cf10k <- do
+        cf <- newCuckooFilter @4 @10 @Int 0 12000
+        traverse_ (insert cf) [1..10000]
+        pure $! cf
+
+    defaultMain
+        [ bgroup "insert"
+            [ insertBench 10000 12000 [1..10000]
+            ]
+        , bgroup "member"
+            [ memberBench "1:1 hit:hiss" 10000 cf10k (interleave [5000..10000] [10001..15000])
+            , memberBench "only hit" 10000 cf10k [1..10000]
+            ]
+#ifdef RANDOM_STANDARD
+        , bgroup "delete"
+            [ deleteBench "all" 10000 cf10k [1..10000]
+            ]
+#endif
+        ]
+
+-- -------------------------------------------------------------------------- --
+
+instance CuckooFilterHash Int
+
+insertBench :: Int -> Int -> [Int] -> Benchmark
+insertBench n cap xs0 = bench ("n-" <> show n <> "-cap-" <> show cap) $ whnf f xs0
+  where
+    f xs = runST $ do
+        cf <- newCuckooFilter @4 @10 @Int 0 (fromIntegral cap)
+        failed <- filterM (fmap not . insert cf) xs
+        pure $! length failed
+
+memberBench :: String -> Int -> CuckooFilter (PrimState IO) 4 10 Int -> [Int] -> Benchmark
+memberBench label n cf xs = bench (label <> " - " <> show n) $ whnfIO f
+  where
+    f = fmap length (filterM (member cf) xs)
+
+#ifdef RANDOM_STANDARD
+deleteBench :: String -> Int -> CuckooFilter (PrimState IO) 4 10 Int -> [Int] -> Benchmark
+deleteBench label n cf0 xs = bench (label <> " - " <> show n) $ whnfIO f
+  where
+    f = do
+        (cf, _) <- splitCuckooFilter cf0
+        fmap length (filterM (delete cf) xs)
+#endif
+
+-- | Doesn't cause actual difference in bench.
+interleave :: [a] -> [a] -> [a]
+interleave (a:as) (b:bs) = a:b:interleave as bs
+interleave as bs = as <> bs
+
diff --git a/cuckoo.cabal b/cuckoo.cabal
index 255480f..6b8e448 100644
--- a/cuckoo.cabal
+++ b/cuckoo.cabal
@@ -70,6 +70,7 @@ library random-internal
           , vector >=0.12
         cpp-options: -DRANDOM_MWC
     else
+        cpp-options: -DRANDOM_STANDARD
         build-depends:
           , random >=1.1
 
@@ -166,3 +167,22 @@ benchmark internal-benchmarks
         , base >=4.10 && <5
         , criterion >= 1.5
 
+benchmark cuckoo-benchmarks
+    type: exitcode-stdio-1.0
+    hs-source-dirs: bench
+    main-is: Cuckoo.hs
+    default-language: Haskell2010
+    ghc-options:
+        -Wall
+        -rtsopts
+        -threaded
+        -with-rtsopts=-N
+    build-depends:
+        -- internal
+        , cuckoo
+
+        -- external
+        , base >=4.10 && <5
+        , criterion >= 1.5
+        , primitive >=0.6.4.0
+
diff --git a/lib/random/System/Random/Internal.hs b/lib/random/System/Random/Internal.hs
index 95c296b..60aed38 100644
--- a/lib/random/System/Random/Internal.hs
+++ b/lib/random/System/Random/Internal.hs
@@ -20,6 +20,7 @@ module System.Random.Internal
 , initialize
 , uniform
 , uniformR
+, genSplit
 ) where
 
 -- -------------------------------------------------------------------------- --
@@ -36,6 +37,12 @@ initialize
     -> m (Gen (PrimState m))
 initialize salt = PCG.initialize 0 (fromIntegral salt)
 
+genSplit
+    :: PrimMonad m
+    => Gen (PrimState m)
+    -> m (Gen (PrimState m), Gen (PrimState m))
+genSplit = error "genSplit: not available in RANDOM_PCG"
+
 -- -------------------------------------------------------------------------- --
 -- MWC
 #elif defined RANDOM_MWC
@@ -51,6 +58,12 @@ initialize
     -> m (Gen (PrimState m))
 initialize salt = MWC.initialize (singleton $ fromIntegral salt)
 
+genSplit
+    :: PrimMonad m
+    => Gen (PrimState m)
+    -> m (Gen (PrimState m), Gen (PrimState m))
+genSplit = error "genSplit: not available in RANDOM_MWC: see https://github.com/haskell/mwc-random/issues/39"
+
 -- -------------------------------------------------------------------------- --
 -- Random
 #else
@@ -95,5 +108,17 @@ uniform gen = stToPrim $ do
     writeSTRef gen g
     return r
 
+-- | Odd name to avoid collision with splitGen.
+-- Using legacy 'split' name for version compatibility.
+genSplit
+    :: PrimMonad m
+    => Gen (PrimState m)
+    -> m (Gen (PrimState m), Gen (PrimState m))
+genSplit gen = stToPrim $ do
+    g <- readSTRef gen
+    let (!g0, !g1) = split g
+    s0 <- newSTRef g0
+    s1 <- newSTRef g1
+    return $ s0 `seq` s1 `seq` (s0, s1)
 #endif
 
diff --git a/src/Data/Cuckoo.hs b/src/Data/Cuckoo.hs
index 7c05bba..a9cff4e 100644
--- a/src/Data/Cuckoo.hs
+++ b/src/Data/Cuckoo.hs
@@ -109,6 +109,9 @@ module Data.Cuckoo
 -- * Debugging Utils
 , showFilter
 , itemHashes
+
+-- * Testing Utils
+, splitCuckooFilter
 ) where
 
 import Control.Applicative
@@ -332,6 +335,24 @@ newCuckooFilter salt n = do
         | not (64 <= n) = error "Seriously? Are you kidding me? If you need to represent such a tiny set, you'll have to pick another data structure for that"
         | otherwise = return ()
 
+-- | Creates two new 'CuckooFilter's with identical data as the original, and with the RNG state split.
+--
+-- This function is not thread-safe. The original CuckooFilter must not be written concurrently during the split operation.
+--
+-- /IMPORTANT/ For testing purposes only - not all random lib choices support RNG state splitting.
+--
+splitCuckooFilter
+    :: PrimMonad m
+    => CuckooFilter (PrimState m) b f a
+    -> m (CuckooFilter (PrimState m) b f a, CuckooFilter (PrimState m) b f a)
+splitCuckooFilter (CuckooFilter bc s rng d) = do
+    len <- getSizeofMutableByteArray d
+    d0 <- cloneMutableByteArray d 0 len
+    d1 <- cloneMutableByteArray d 0 len
+    (g0, g1) <- genSplit rng
+    return $! (CuckooFilter bc s g0 d0, CuckooFilter bc s g1 d1)
+
+
 -- -------------------------------------------------------------------------- --
 -- Insert
 
@@ -356,6 +377,11 @@ newCuckooFilter salt n = do
 -- function. If this function is used in the presence of asynchronous exceptions
 -- it should be apprioriately masked.
 --
+-- Note: if the potentially inserted elements have many repetitions, and you
+-- don't intend to delete items, rather just use the filter as a set, then it is
+-- advised to perform a 'member' check before insert. Otherwise repeated items
+-- will take up slots and cause high load or even premature insertion failure.
+--
 -- >>> f <- newCuckooFilter @4 @10 @Int 0 1000
 -- >>> insert f 0
 -- True
@@ -401,6 +427,7 @@ insert f a = do
         setFingerprint f b i k
         return k'
     {-# INLINE swapFingerprint #-}
+{-# INLINABLE insert #-}
 
 -- -------------------------------------------------------------------------- --
 -- Member Test
@@ -492,6 +519,7 @@ delete f a = do
         Nothing -> checkBucket f b2 fp >>= \case
             Just i -> True <$ setFingerprint f b2 i null
             Nothing -> return False
+{-# INLINABLE delete #-}
 
 -- -------------------------------------------------------------------------- --
 -- Internal