# UNDF: UNDF-2026-000000035
diff --git a/Source/cmTarget.cxx b/Source/cmTarget.cxx
index abc1234..def5678 100644
--- a/Source/cmTarget.cxx
+++ b/Source/cmTarget.cxx
@@ -1428,11 +1428,19 @@ cmSourceFile* cmTarget::AddSource(std::string const& src, bool before)
 {
   cmSourceFileLocation sfl(this->impl->Makefile, src,
                            cmSourceFileLocationKind::Known);
+  // Fast path: check a parallel unordered_set of source-path strings.
+  // The full TargetPropertyEntryFinder scan is O(n) and is called from loops
+  // (e.g. Unity build file iteration), producing O(n^2) overall.
+  // An exact-string set lookup is O(1) amortized and covers the common case
+  // where src is a canonical resolved path.
+  if (!this->impl->SourcePathsSet.insert(src).second) {
+    // Already present — skip the expensive find_if and WriteDirect.
+    if (cmGeneratorExpression::Find(src) != std::string::npos)
+      return nullptr;
+    return this->impl->Makefile->GetOrCreateSource(
+      src, false, cmSourceFileLocationKind::Known);
+  }
   auto const& sources = this->impl->Sources.Entries;
   if (std::find_if(sources.begin(), sources.end(),
                    TargetPropertyEntryFinder(sfl)) == sources.end()) {
     this->impl->Sources.WriteDirect(
       this->impl.get(), {}, cmValue(src),
       before ? UsageRequirementProperty::Action::Prepend
              : UsageRequirementProperty::Action::Append);
   }