diff --git a/bench/Bench.hs b/bench/Bench.hs
--- a/bench/Bench.hs
+++ b/bench/Bench.hs
@@ -2,11 +2,12 @@
 
 module Main (main) where
 
+import Prelude hiding (Foldable(..))
 #ifdef MIN_VERSION_digits
 import qualified Data.Digits as D (digitsRev)
 #endif
 import Data.FastDigits (digits, undigits)
-import Data.List (foldl')
+import Data.Foldable
 import Test.Tasty.Bench (Benchmark, Benchmarkable, defaultMain, bench, bgroup, nf)
 #ifdef MIN_VERSION_digits
 import Test.Tasty.Bench (bcompare)
@@ -67,4 +68,9 @@
   [ benchSmth "short"  benchShort
   , benchSmth "medium" benchMedium
   , benchSmth "long"   benchLong
+
+  , bgroup "undigits"
+  $ map
+    (\n -> bench (show (n :: Int)) $ nf (\m -> undigits m [1..m-1]) n)
+    [10, 100, 1000, 10000, 100000]
   ]
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,7 @@
+# 0.3.3.0
+
+* Avoid quadratic complexity in `undigits`.
+
 # 0.3.2.0
 
 * Migrate from `integer-gmp` to `ghc-bignum`.
diff --git a/fast-digits.cabal b/fast-digits.cabal
--- a/fast-digits.cabal
+++ b/fast-digits.cabal
@@ -1,5 +1,5 @@
 name: fast-digits
-version: 0.3.2.0
+version: 0.3.3.0
 license: GPL-3
 license-file: LICENSE
 maintainer: andrew.lelechenko@gmail.com
@@ -18,11 +18,11 @@
   changelog.md
   README.md
 tested-with:
-  GHC ==9.0.2 GHC ==9.2.8 GHC ==9.4.5 GHC ==9.6.2
+  GHC ==9.0.2 GHC ==9.2.8 GHC ==9.4.8 GHC ==9.6.7 GHC ==9.8.4 GHC ==9.10.3 GHC ==9.12.2 GHC ==9.14.1
 
 source-repository head
   type: git
-  location: git://github.com/Bodigrim/fast-digits.git
+  location: https://github.com/Bodigrim/fast-digits
 
 library
   exposed-modules:
@@ -32,7 +32,7 @@
   ghc-options: -Wall -O2 -Wcompat
   build-depends:
     base >=4.15 && <5,
-    ghc-bignum <1.4,
+    ghc-bignum <1.5,
     fast-digits-internal
 
 library fast-digits-internal
@@ -52,10 +52,9 @@
   ghc-options: -Wall -Wcompat
   build-depends:
     base,
-    tasty <1.5,
-    tasty-quickcheck <0.11,
+    tasty <1.6,
+    tasty-quickcheck <0.12,
     tasty-smallcheck <0.9,
-    QuickCheck <2.15,
     smallcheck <1.3,
     -- digits,
     fast-digits,
@@ -71,4 +70,4 @@
     base,
     -- digits,
     fast-digits,
-    tasty-bench >= 0.2.4 && <0.4
+    tasty-bench >= 0.4 && <0.6
diff --git a/src/Data/FastDigits.hs b/src/Data/FastDigits.hs
--- a/src/Data/FastDigits.hs
+++ b/src/Data/FastDigits.hs
@@ -145,9 +145,22 @@
          => a       -- ^ The base to use
          -> [b]     -- ^ The list of digits to convert
          -> Integer
-undigits base' = foldr (\d acc -> acc * base + toInteger d) 0
-  where
-    base = toInteger base'
+undigits base xs
+  | length xs < 700 = undigitsSmall (toInteger base) xs
+  | otherwise = undigitsHuge (toInteger base) xs
 {-# SPECIALIZE undigits :: Word    -> [Word]    -> Integer #-}
 {-# SPECIALIZE undigits :: Int     -> [Int]     -> Integer #-}
 {-# SPECIALIZE undigits :: Integer -> [Integer] -> Integer #-}
+
+undigitsSmall :: Integral b => Integer -> [b] -> Integer
+undigitsSmall base = foldr (\d acc -> acc * base + toInteger d) 0
+
+undigitsHuge :: Integral b => Integer -> [b] -> Integer
+undigitsHuge !_ [] = 0
+undigitsHuge base (x : xs) = go (toInteger x) 1 base xs
+  where
+    go acc power poweredBase rest = case splitAt power rest of
+      ([], _) -> acc
+      (p : pref, rest') -> go acc' (power * 2) (poweredBase ^ (2 :: Int)) rest'
+        where
+          acc' = acc + poweredBase * go (toInteger p) 1 base pref
diff --git a/tests/Tests.hs b/tests/Tests.hs
--- a/tests/Tests.hs
+++ b/tests/Tests.hs
@@ -8,6 +8,10 @@
 import Test.Tasty.SmallCheck as SC (testProperty)
 import Test.Tasty.QuickCheck as QC (Positive(..), NonNegative(..), Property, (==>), (===), testProperty)
 
+#if MIN_VERSION_base(4,19,0)
+import Data.List (unsnoc)
+#endif
+
 #ifdef MIN_VERSION_digits
 import qualified Data.Digits as D (digitsRev, unDigits)
 #endif
@@ -70,11 +74,15 @@
 -- Last digit is not 0
 qProperty5 :: QC.Positive Int -> [QC.Positive Int] -> QC.Property
 qProperty5 (QC.Positive base) (largeInteger -> QC.NonNegative n) = base /= 1 && n /= 0 QC.==>
-  ((/= 0) $ last $ digits base n)
+  case unsnoc (digits base n) of
+    Nothing -> False
+    Just (_, d) -> d /= 0
 
 sProperty5 :: SC.Positive Int -> SC.Positive Integer -> Bool
 sProperty5 (SC.Positive base) (SC.Positive n) = base == 1 ||
-  ((/= 0) $ last $ digits base n)
+  case unsnoc (digits base n) of
+    Nothing -> False
+    Just (_, d) -> d /= 0
 
 #ifdef MIN_VERSION_digits
 -- digits 2 == digitsD 2
@@ -140,3 +148,8 @@
 
 main :: IO ()
 main = defaultMain testSuite
+
+#if !MIN_VERSION_base(4,19,0)
+unsnoc :: [a] -> Maybe ([a], a)
+unsnoc = foldr (\x -> Just . maybe ([], x) (\(~(a, b)) -> (x : a, b))) Nothing
+#endif