Add release notes for PR #681

docs/release notes/4.1.0/681.performance.md

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+## Performance Improvements: Vector Serialization
+
+### Summary
+Optimized vector serialization in `MemorySegmentVectorProvider` by replacing
+per-element scalar writes with bulk operations, significantly reducing
+serialization overhead.
+
+### Key Changes
+- **`writeFloatVector`**
+  - Uses backing `float[]` with `writeFloats()` instead of per-element
+    `writeFloat()`
+
+- **`writeByteSequence`**
+  - Uses backing `byte[]` with bulk `write()` instead of per-element
+    `writeByte()`
+
+### Impact
+- Removes a major scalar-write bottleneck in the native SIMD backend
+- Improves index construction performance on large Intel hosts (e.g., AWS
+  `x8i.24xlarge`)
+
+### Index Build Improvements
+- **openai-1536-1m:** ~57% faster (105.59s → 45.22s)
+- **openai-3072-1m:** ~38% faster (164.63s → 102.75s)
+
+### Microbenchmark Summary
+- Bulk serialization improves throughput by **~1.3× to 1.9×** across tested
+  vector sizes