docs: add performance and thread safety entries to 3.1.0 release notes

Author: fgmacedo

docs/releases/3.1.0.md

@@ -33,6 +33,22 @@ See {ref}`diagram:Sphinx directive` for full documentation.
 [#589](https://github.com/fgmacedo/python-statemachine/pull/589).
 
 
+### Performance: 5x–7x faster event processing
+
+The engine's hot paths have been systematically profiled and optimized, resulting in
+**4.7x–7.7x faster event throughput** and **1.9x–2.6x faster setup** across all
+machine types. All optimizations are internal — no public API changes.
+See [#592](https://github.com/fgmacedo/python-statemachine/pull/592) for details.
+
+
+### Thread safety documentation
+
+The sync engine is thread-safe: multiple threads can send events to the same state
+machine instance concurrently. This is now documented in the
+{ref}`processing model <thread-safety>` and verified by stress tests.
+[#592](https://github.com/fgmacedo/python-statemachine/pull/592).
+
+
 ### Bugfixes in 3.1.0
 
 - Fixes silent misuse of `Event()` with multiple positional arguments. Passing more than one