diff --git a/src/ShellCheck/CFG.hs b/src/ShellCheck/CFG.hs
index 39747cf..771e870 100644
--- a/src/ShellCheck/CFG.hs
+++ b/src/ShellCheck/CFG.hs
@@ -47,6 +47,8 @@ import ShellCheck.Regex
 import Control.DeepSeq
 import Control.Monad
 import Control.Monad.Identity
+import Data.Array.Unboxed
+import Data.Array.ST
 import Data.List hiding (map)
 import Data.Maybe
 import qualified Data.Map as M
@@ -174,10 +176,10 @@ data CFGResult = CFGResult {
     cfIdToRange :: M.Map Id (Node, Node),
     -- A set of all nodes belonging to an Id, recursively
     cfIdToNodes :: M.Map Id (S.Set Node),
-    -- A map to nodes that the given node postdominates
-    cfPostDominators :: M.Map Node (S.Set Node)
+    -- An array (from,to) saying whether 'from' postdominates 'to'
+    cfPostDominators :: Array Node [Node]
 }
-  deriving (Show, Generic, NFData)
+  deriving (Show)
 
 buildGraph :: CFGParameters -> Token -> CFGResult
 buildGraph params root =
@@ -200,7 +202,7 @@ buildGraph params root =
             cfPostDominators = findPostDominators mainExit $ mkGraph nodes onlyRealEdges
         }
     in
-        deepseq result result
+        result
 
 remapGraph :: M.Map Node Node -> CFW -> CFW
 remapGraph remap (nodes, edges, mapping, assoc) =
@@ -1256,8 +1258,8 @@ findTerminalNodes graph = ufold find [] graph
     f (IdTagged _ (CFDefineFunction _ id start end):rest) list = f rest (end:list)
     f (_:rest) list = f rest list
 
-findPostDominators :: Node -> CFGraph -> M.Map Node (S.Set Node)
-findPostDominators mainexit graph = asSetMap
+findPostDominators :: Node -> CFGraph -> Array Node [Node]
+findPostDominators mainexit graph = asArray
   where
     inlined = inlineSubshells graph
     terminals = findTerminalNodes inlined
@@ -1265,7 +1267,8 @@ findPostDominators mainexit graph = asSetMap
     withExitEdges = (incoming ++ map (\c -> (CFEFlow, c)) terminals, mainexit, label, outgoing) `safeUpdate` inlined
     reversed = grev withExitEdges
     postDoms = dom reversed mainexit
-    asSetMap = M.fromList $ map (\(node, list) -> (node, S.fromList list)) postDoms
+    (_, maxNode) = nodeRange graph
+    asArray = array (0, maxNode) postDoms
 
 return []
 runTests =  $( [| $(forAllProperties) (quickCheckWithResult (stdArgs { maxSuccess = 1 }) ) |])
diff --git a/src/ShellCheck/CFGAnalysis.hs b/src/ShellCheck/CFGAnalysis.hs
index ff88810..e6b1701 100644
--- a/src/ShellCheck/CFGAnalysis.hs
+++ b/src/ShellCheck/CFGAnalysis.hs
@@ -69,6 +69,7 @@ import Control.Monad
 import Control.Monad.ST
 import Control.DeepSeq
 import Data.List hiding (map)
+import Data.Array.Unboxed
 import Data.STRef
 import Data.Maybe
 import qualified Data.Map as M
@@ -100,9 +101,9 @@ data CFGAnalysis = CFGAnalysis {
     graph :: CFGraph,
     tokenToRange :: M.Map Id (Node, Node),
     tokenToNodes :: M.Map Id (S.Set Node),
-    postDominators :: M.Map Node (S.Set Node),
+    postDominators :: Array Node [Node],
     nodeToData :: M.Map Node (ProgramState, ProgramState)
-} deriving (Show, Generic, NFData)
+} deriving (Show)
 
 -- The program state we expose externally
 data ProgramState = ProgramState {
@@ -147,8 +148,7 @@ doesPostDominate :: CFGAnalysis -> Id -> Id -> Bool
 doesPostDominate analysis target base = fromMaybe False $ do
     (_, baseEnd) <- M.lookup base $ tokenToRange analysis
     (targetStart, _) <- M.lookup target $ tokenToRange analysis
-    postDoms <- M.lookup baseEnd $ postDominators analysis
-    return $ S.member targetStart postDoms
+    return $ targetStart `elem` (postDominators analysis ! baseEnd)
 
 getDataForNode analysis node = M.lookup node $ nodeToData analysis