diff --git a/lld/COFF/DebugTypes.h b/lld/COFF/DebugTypes.h
--- a/lld/COFF/DebugTypes.h
+++ b/lld/COFF/DebugTypes.h
@@ -72,6 +72,9 @@
   void remapRecord(MutableArrayRef<uint8_t> rec,
                    ArrayRef<llvm::codeview::TiReference> typeRefs);
 
+  void computeTypeRecordSize(llvm::codeview::CVType ty, unsigned &nbTpiRecs,
+                             unsigned &nbIpiRecs);
+
   void mergeTypeRecord(TypeIndex curIndex, llvm::codeview::CVType ty);
 
   // Merge the type records listed in uniqueTypes. beginIndex is the TypeIndex
diff --git a/lld/COFF/DebugTypes.cpp b/lld/COFF/DebugTypes.cpp
--- a/lld/COFF/DebugTypes.cpp
+++ b/lld/COFF/DebugTypes.cpp
@@ -620,6 +620,17 @@
   });
 }
 
+void TpiSource::computeTypeRecordSize(CVType ty, unsigned &nbTpiRecs,
+                                      unsigned &nbIpiRecs) {
+  assert(ty.length() <= codeview::MaxRecordLength);
+  size_t newSize = alignTo(ty.length(), 4);
+
+  // Decide if the merged type goes into TPI or IPI.
+  bool isItem = isIdRecord(ty.kind());
+
+  (isItem ? nbIpiRecs : nbTpiRecs) += newSize;
+}
+
 void TpiSource::mergeTypeRecord(TypeIndex curIndex, CVType ty) {
   // Decide if the merged type goes into TPI or IPI.
   bool isItem = isIdRecord(ty.kind());
@@ -679,6 +690,24 @@
   auto nextUniqueIndex = uniqueTypes.begin();
   assert(mergedTpi.recs.empty());
   assert(mergedIpi.recs.empty());
+
+  // Pre-compute the number of elements in advance to avoid std::vector resizes.
+  unsigned nbTpiRecs = 0;
+  unsigned nbIpiRecs = 0;
+  forEachTypeChecked(typeRecords, [&](const CVType &ty) {
+    if (nextUniqueIndex != uniqueTypes.end() &&
+        *nextUniqueIndex == ghashIndex) {
+      computeTypeRecordSize(ty, nbTpiRecs, nbIpiRecs);
+      ++nextUniqueIndex;
+    }
+    ++ghashIndex;
+  });
+  mergedTpi.recs.reserve(nbTpiRecs);
+  mergedIpi.recs.reserve(nbIpiRecs);
+
+  // Do the actual type merge.
+  ghashIndex = 0;
+  nextUniqueIndex = uniqueTypes.begin();
   forEachTypeChecked(typeRecords, [&](const CVType &ty) {
     if (nextUniqueIndex != uniqueTypes.end() &&
         *nextUniqueIndex == ghashIndex) {