diff --git a/Changes b/Changes
index bc886515c..843fc5ae2 100644
--- a/Changes
+++ b/Changes
@@ -24,6 +24,7 @@ The contributors that suggested a given feature are shown in []. Thanks!
 
 ****  Fix parsing error on bad missing #, bug1308. [Dan Kirkham]
 
+****  Fix Verilation performance issues, bug1316. [John Coiner]
 
 * Verilator 3.922 2018-03-17
 
diff --git a/src/V3Active.cpp b/src/V3Active.cpp
index bf50c350c..23d78cf3c 100644
--- a/src/V3Active.cpp
+++ b/src/V3Active.cpp
@@ -44,6 +44,8 @@
 #include "V3Ast.h"
 #include "V3EmitCBase.h"
 #include "V3Const.h"
+#include "V3SenTree.h"  // for SenTreeSet
+#include VL_INCLUDE_UNORDERED_MAP
 
 //***** See below for main transformation engine
 
@@ -62,7 +64,11 @@ private:
     AstScope*	m_scopep;		// Current scope to add statement to
     AstActive*	m_iActivep;		// For current scope, the IActive we're building
     AstActive*	m_cActivep;		// For current scope, the SActive(combo) we're building
-    std::vector<AstActive*> m_activeVec;  // List of sensitive actives, for folding
+
+    SenTreeSet  m_activeSens;  // Sen lists for each active we've made
+    typedef vl_unordered_map<AstSenTree*, AstActive*> ActiveMap;
+    ActiveMap m_activeMap; // Map sentree to active, for folding.
+
     // METHODS
     void addActive(AstActive* nodep) {
 	if (!m_scopep) nodep->v3fatalSrc("NULL scope");
@@ -73,7 +79,8 @@ private:
 	m_scopep = nodep;
 	m_iActivep = NULL;
 	m_cActivep = NULL;
-	m_activeVec.clear();
+        m_activeSens.clear();
+        m_activeMap.clear();
         iterateChildren(nodep);
 	// Don't clear scopep, the namer persists beyond this visit
     }
@@ -112,22 +119,15 @@ public:
     }
     AstActive* getActive(FileLine* fl, AstSenTree* sensesp) {
 	// Return a sentree in this scope that matches given sense list.
-	// Not the fastest, but scopes tend to have few clocks
+
 	AstActive* activep = NULL;
-	//sitemsp->dumpTree(cout,"  Lookingfor: ");
-        for (std::vector<AstActive*>::iterator it = m_activeVec.begin(); it!=m_activeVec.end(); ++it) {
-	    activep = *it;
-	    if (activep) {  // Not deleted
-		// Compare the list
-		AstSenTree* asenp = activep->sensesp();
-		if (asenp->sameTree(sensesp)) {
-		    UINFO(8,"    Found ACTIVE "<<activep<<endl);
-		    goto found;
-		}
-	    }
-	    activep = NULL;
-	}
-      found:
+        AstSenTree* activeSenp = m_activeSens.find(sensesp);
+        if (activeSenp) {
+            ActiveMap::iterator it = m_activeMap.find(activeSenp);
+            UASSERT(it != m_activeMap.end(), "Corrupt active map");
+            activep = it->second;
+        }
+
 	// Not found, form a new one
 	if (!activep) {
 	    AstSenTree* newsenp = sensesp->cloneTree(false);
@@ -136,7 +136,8 @@ public:
 	    UINFO(8,"    New ACTIVE "<<activep<<endl);
 	    // Form the sensitivity list
 	    addActive(activep);
-	    m_activeVec.push_back(activep);
+            m_activeMap[newsenp] = activep;
+            m_activeSens.add(newsenp);
 	    // Note actives may have also been added above in the Active visitor
 	}
 	return activep;
diff --git a/src/V3AstNodes.h b/src/V3AstNodes.h
index 00ce3faed..a2e775a78 100644
--- a/src/V3AstNodes.h
+++ b/src/V3AstNodes.h
@@ -2120,6 +2120,7 @@ public:
     ASTNODE_NODE_FUNCS(SenTree)
     virtual void dump(std::ostream& str);
     virtual bool maybePointedTo() const { return true; }
+    virtual V3Hash sameHash() const { return V3Hash(); }
     bool isMulti() const { return m_multi; }
     AstNodeSenItem* sensesp() const { return VN_CAST(op1p(), NodeSenItem); }  // op1 = Sensitivity list
     void addSensesp(AstNodeSenItem* nodep) { addOp1p(nodep); }
diff --git a/src/V3Delayed.cpp b/src/V3Delayed.cpp
index 9fbe257c1..d6df46564 100644
--- a/src/V3Delayed.cpp
+++ b/src/V3Delayed.cpp
@@ -146,7 +146,8 @@ private:
     AstActive* createActivePost(AstVarRef* varrefp) {
 	AstActive* newactp = new AstActive (varrefp->fileline(), "sequentdly",
 					    m_activep->sensesp());
-	m_activep->addNext(newactp);
+        // Was addNext(), but addNextHere() avoids a linear search.
+        m_activep->addNextHere(newactp);
 	return newactp;
     }
     void checkActivePost(AstVarRef* varrefp, AstActive* oldactivep) {
diff --git a/src/V3Hashed.cpp b/src/V3Hashed.cpp
index 36d2fefb6..4069acaa4 100644
--- a/src/V3Hashed.cpp
+++ b/src/V3Hashed.cpp
@@ -52,17 +52,19 @@ private:
 
     // STATE
     V3Hash		m_lowerHash;	// Hash of the statement we're building
+    bool                m_cacheInUser4; // Use user4 to cache each V3Hash?
 
     // METHODS
     VL_DEBUG_FUNC;  // Declare debug()
 
     void nodeHashIterate(AstNode* nodep) {
-	if (!nodep->user4()) {
+        V3Hash thisHash;
+        if (!m_cacheInUser4 || !nodep->user4()) {
             if (VN_IS(nodep->backp(), CFunc)
                 && !(VN_IS(nodep, NodeStmt) || VN_IS(nodep, CFunc))) {
 		nodep->v3fatalSrc("Node "<<nodep->prettyTypeName()<<" in statement position but not marked stmt (node under function)");
 	    }
-	    V3Hash oldHash = m_lowerHash;
+            V3Hash oldHash = m_lowerHash;
 	    {
 		m_lowerHash = nodep->sameHash();
 		if (m_lowerHash.isIllegal()) {
@@ -76,10 +78,12 @@ private:
 		nodep->user4(m_lowerHash.fullValue());
 		//UINFO(9, "    hashnode "<<m_lowerHash<<"  "<<nodep<<endl);
 	    }
+            thisHash = m_lowerHash;
 	    m_lowerHash = oldHash;
 	}
 	// Update what will become the above node's hash
-	m_lowerHash += V3Hashed::nodeHash(nodep);
+        m_lowerHash += m_cacheInUser4
+            ? V3Hashed::nodeHash(nodep) : thisHash;
     }
 
     //--------------------
@@ -94,15 +98,26 @@ private:
 public:
     // CONSTUCTORS
     explicit HashedVisitor(AstNode* nodep) {
+        m_cacheInUser4 = true;
 	nodeHashIterate(nodep);
 	//UINFO(9,"  stmthash "<<hex<<V3Hashed::nodeHash(nodep)<<"  "<<nodep<<endl);
     }
+    explicit HashedVisitor(const AstNode* nodep) {
+        m_cacheInUser4 = false;
+        nodeHashIterate(const_cast<AstNode*>(nodep));
+    }
+    V3Hash finalHash() const { return m_lowerHash; }
     virtual ~HashedVisitor() {}
 };
 
 //######################################################################
 // Hashed class functions
 
+V3Hash V3Hashed::uncachedHash(const AstNode* nodep) {
+    HashedVisitor visitor(nodep);
+    return visitor.finalHash();
+}
+
 V3Hashed::iterator V3Hashed::hashAndInsert(AstNode* nodep) {
     hash(nodep);
     return m_hashMmap.insert(make_pair(nodeHash(nodep), nodep));
diff --git a/src/V3Hashed.h b/src/V3Hashed.h
index 7a2c157e5..d020a4d85 100644
--- a/src/V3Hashed.h
+++ b/src/V3Hashed.h
@@ -83,6 +83,8 @@ public:
     void dumpFile(const string& filename, bool tree);
     void dumpFilePrefixed(const string& nameComment, bool tree=false);
     static V3Hash nodeHash(AstNode* nodep) { return V3Hash(nodep->user4p()); }
+    // Hash of the nodep tree, without caching in user4.
+    static V3Hash uncachedHash(const AstNode* nodep);
 };
 
 #endif // Guard
diff --git a/src/V3SenTree.h b/src/V3SenTree.h
index f27177819..49dfab33a 100644
--- a/src/V3SenTree.h
+++ b/src/V3SenTree.h
@@ -41,19 +41,69 @@
 #include <map>
 #include <algorithm>
 #include <vector>
+#include VL_INCLUDE_UNORDERED_SET
 
 #include "V3Global.h"
 #include "V3Ast.h"
+#include "V3Hashed.h"
 
 //######################################################################
 // Collect SenTrees under the entire scope
 // And provide functions to find/add a new one
 
+class SenTreeSet {
+    // Hash table of sensitive blocks.
+private:
+    // TYPES
+    class HashSenTree {
+    public:
+        HashSenTree() {}
+        size_t operator() (const AstSenTree* kp) const {
+            return V3Hashed::uncachedHash(kp).fullValue();
+        }
+    private:
+        VL_UNCOPYABLE(HashSenTree);
+    };
+
+    class EqSenTree {
+    public:
+        EqSenTree() {}
+        bool operator() (const AstSenTree* ap, const AstSenTree* bp) const {
+            return ap->sameTree(bp);
+        }
+    private:
+        VL_UNCOPYABLE(EqSenTree);
+    };
+
+    // MEMBERS
+    typedef vl_unordered_set<AstSenTree*, HashSenTree, EqSenTree> Set;
+    Set m_trees;  // Set of sensitive blocks, for folding.
+
+public:
+    // CONSTRUCTORS
+    SenTreeSet() {}
+
+    // METHODS
+    void add(AstSenTree* nodep) { m_trees.insert(nodep); }
+    AstSenTree* find(AstSenTree* likep) {
+        AstSenTree* resultp = NULL;
+        Set::iterator it = m_trees.find(likep);
+        if (it != m_trees.end()) {
+            resultp = *it;
+        }
+        return resultp;
+    }
+    void clear() { m_trees.clear(); }
+
+private:
+    VL_UNCOPYABLE(SenTreeSet);
+};
+
 class SenTreeFinder : public AstNVisitor {
 private:
     // STATE
-    AstTopScope*	m_topscopep;		// Top scope to add statement to
-    std::vector<AstSenTree*> m_treesp;  // List of sensitive blocks, for folding
+    AstTopScope*        m_topscopep;    // Top scope to add statement to
+    SenTreeSet m_trees;  // Set of sensitive blocks, for folding
 
     // VISITORS
     VL_DEBUG_FUNC;  // Declare debug()
@@ -77,9 +127,7 @@ private:
 	// Don't grab SenTrees under Actives, only those that are global (under Scope directly)
         iterateChildren(nodep);
     }
-    virtual void visit(AstSenTree* nodep) {
-	m_treesp.push_back(nodep);
-    }
+    virtual void visit(AstSenTree* nodep) { m_trees.add(nodep); }
     // Empty visitors, speed things up
     virtual void visit(AstNodeStmt* nodep) { }
     virtual void visit(AstNode* nodep) {
@@ -89,31 +137,18 @@ private:
 public:
     void clear() {
 	m_topscopep = NULL;
-	m_treesp.clear();
+        m_trees.clear();
     }
     AstSenTree* getSenTree(FileLine* fl, AstSenTree* sensesp) {
 	// Return a global sentree that matches given sense list.
-	// Not the fastest, but there tend to be few clocks
-	AstSenTree* treep = NULL;
-	//sensesp->dumpTree(cout,"  Lookingfor: ");
-        for (std::vector<AstSenTree*>::iterator it = m_treesp.begin(); it!=m_treesp.end(); ++it) {
-	    treep = *it;
-	    if (treep) {  // Not deleted
-		if (treep->sameTree(sensesp)) {
-		    UINFO(8,"    Found SBLOCK "<<treep<<endl);
-		    goto found;
-		}
-	    }
-	    treep = NULL;
-	}
-      found:
+        AstSenTree* treep = m_trees.find(sensesp);
 	// Not found, form a new one
 	if (!treep) {
 	    UASSERT(m_topscopep,"Never called main()");
 	    treep = sensesp->cloneTree(false);
 	    m_topscopep->addStmtsp(treep);
 	    UINFO(8,"    New SENTREE "<<treep<<endl);
-	    m_treesp.push_back(treep);
+            m_trees.add(treep);
 	    // Note blocks may have also been added above in the Active visitor
 	}
 	return treep;