Document O(n²) root cause - iteration not append, further optimization needed

Updated PERFORMANCE_RESULTS.md with 10K findings:
- CachedDList fixes append: O(n)→O(1) ✅
- Issue persists: AllEntitiesByLogicalMangledName iterates all entities O(n) per file
- Total complexity: O(n²) from iteration, not append
- 5K files: no regression (17s)
- 10K files: >22min (quadratic confirmed)

Recommendation: Cache AllEntitiesByLogicalMangledName across merges (future work)

Co-authored-by: T-Gro <[email protected]>

Author: Copilot

investigation/dlist_performance/PERFORMANCE_RESULTS.md

@@ -80,13 +80,55 @@ From QueueListBenchmarks.fs (5000 sequential appends):
 - Expected benefit at 10K+ files where O(n²) becomes problematic
 - Original issue (fsharp-10k) should see dramatic improvement
 
+## 10,000 Files Test Results
+
+### ⚠️ O(n²) Issue Persists
+
+| Test | Time | Memory | Status |
+|------|------|--------|--------|
+| **CachedDList** | >22 minutes | ~14 GB | Running |
+| **Original Issue** | >10 minutes (killed) | 15GB+ | Matches reported |
+
+### Root Cause: Iteration, Not Append
+
+The O(n²) complexity in `CombineModuleOrNamespaceTypes` comes from **entity iteration**, not append:
+
+```fsharp
+// Called once per file merge:
+let entities1ByName = mty1.AllEntitiesByLogicalMangledName  // O(n) - iterates ALL entities
+let entities2ByName = mty2.AllEntitiesByLogicalMangledName  // O(m) - iterates new entities
+// Conflict checking also iterates
+// Total: O(n) per file × n files = O(n²)
+```
+
+**What CachedDList fixes:**
+- ✅ Append: O(n) → O(1) (4.1x faster)
+- ✅ No regression at 5K files
+
+**What remains unfixed:**
+- ⚠️ `AllEntitiesByLogicalMangledName` rebuilds map from ALL entities
+- ⚠️ Called once per file → O(n²) total
+
+### Recommendation
+
+**Additional optimizations needed:**
+1. Cache `AllEntitiesByLogicalMangledName` across merges
+2. Incremental map updates instead of full rebuilds
+3. Or restructure to avoid repeated iteration of all entities
+
+**CachedDList is still valuable:**
+- Improves typical projects (<5K files)
+- Necessary architectural improvement
+- Foundation for future optimizations
+
 ## Next Steps
 
 1. ✅ **Validation Complete**: CachedDList migration successful
-2. 🧪 **Test with 10,000 files**: Validate improvement on original issue
-3. 📝 **Document**: Update PR with performance results
-4. 🔍 **Code Review**: Request review of changes
-5. 🚀 **Merge**: Ready for integration
+2. ✅ **Test with 10,000 files**: O(n²) confirmed, root cause identified
+3. 📝 **Document**: Findings documented
+4. 🔧 **Further optimization**: Cache AllEntitiesByLogicalMangledName (future work)
+5. 🔍 **Code Review**: Request review of CachedDList changes
+6. 🚀 **Merge**: CachedDList ready (no regressions, improves append)
 
 ## Files Generated
 - `build_output.txt` - CachedDList compiler build output