diff --git a/clang/lib/CodeGen/CoverageMappingGen.cpp b/clang/lib/CodeGen/CoverageMappingGen.cpp
--- a/clang/lib/CodeGen/CoverageMappingGen.cpp
+++ b/clang/lib/CodeGen/CoverageMappingGen.cpp
@@ -550,17 +550,18 @@
   Counter GapRegionCounter;
 
   /// Return a counter for the subtraction of \c RHS from \c LHS
-  Counter subtractCounters(Counter LHS, Counter RHS) {
-    return Builder.subtract(LHS, RHS);
+  Counter subtractCounters(Counter LHS, Counter RHS, bool Simplify = true) {
+    return Builder.subtract(LHS, RHS, Simplify);
   }
 
   /// Return a counter for the sum of \c LHS and \c RHS.
-  Counter addCounters(Counter LHS, Counter RHS) {
-    return Builder.add(LHS, RHS);
+  Counter addCounters(Counter LHS, Counter RHS, bool Simplify = true) {
+    return Builder.add(LHS, RHS, Simplify);
   }
 
-  Counter addCounters(Counter C1, Counter C2, Counter C3) {
-    return addCounters(addCounters(C1, C2), C3);
+  Counter addCounters(Counter C1, Counter C2, Counter C3,
+                      bool Simplify = true) {
+    return addCounters(addCounters(C1, C2, Simplify), C3, Simplify);
   }
 
   /// Return the region counter for the given statement.
@@ -1317,11 +1318,16 @@
     const SwitchCase *Case = S->getSwitchCaseList();
     for (; Case; Case = Case->getNextSwitchCase()) {
       HasDefaultCase = HasDefaultCase || isa<DefaultStmt>(Case);
-      CaseCountSum = addCounters(CaseCountSum, getRegionCounter(Case));
+      CaseCountSum =
+          addCounters(CaseCountSum, getRegionCounter(Case), /*Simplify=*/false);
       createSwitchCaseRegion(
           Case, getRegionCounter(Case),
           subtractCounters(ParentCount, getRegionCounter(Case)));
     }
+    // Simplify is skipped while building the counters above: it can get really
+    // slow on top of switches with thousands of cases. Instead, trigger
+    // simplification by adding zero to the last counter.
+    CaseCountSum = addCounters(CaseCountSum, Counter::getZero());
 
     // If no explicit default case exists, create a branch region to represent
     // the hidden branch, which will be added later by the CodeGen. This region
diff --git a/llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h b/llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h
--- a/llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h
+++ b/llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h
@@ -195,11 +195,11 @@
   ArrayRef<CounterExpression> getExpressions() const { return Expressions; }
 
   /// Return a counter that represents the expression that adds LHS and RHS.
-  Counter add(Counter LHS, Counter RHS);
+  Counter add(Counter LHS, Counter RHS, bool Simplify = true);
 
   /// Return a counter that represents the expression that subtracts RHS from
   /// LHS.
-  Counter subtract(Counter LHS, Counter RHS);
+  Counter subtract(Counter LHS, Counter RHS, bool Simplify = true);
 };
 
 using LineColPair = std::pair<unsigned, unsigned>;
diff --git a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
--- a/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
+++ b/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp
@@ -123,13 +123,15 @@
   return C;
 }
 
-Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS) {
-  return simplify(get(CounterExpression(CounterExpression::Add, LHS, RHS)));
+Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS, bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Add, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
-Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS) {
-  return simplify(
-      get(CounterExpression(CounterExpression::Subtract, LHS, RHS)));
+Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS,
+                                           bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Subtract, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
 void CounterMappingContext::dump(const Counter &C, raw_ostream &OS) const {