[docs] Update common_issues.rst: mention orjson in the Mypy slow section (#19058)

This mostly just copies the verbiage from the release blog post.

Author: wyattscarpenter

docs/source/common_issues.rst

@@ -218,6 +218,14 @@ daemon <mypy_daemon>`, which can speed up incremental mypy runtimes by
 a factor of 10 or more. :ref:`Remote caching <remote-cache>` can
 make cold mypy runs several times faster.
 
+Furthermore: as of `mypy 1.13 <https://mypy-lang.blogspot.com/2024/10/mypy-113-released.html>`_,
+mypy allows use of the orjson library for handling the cache instead of the stdlib json, for
+improved performance. You can ensure the presence of orjson using the faster-cache extra:
+
+    python3 -m pip install -U mypy[faster-cache]
+
+Mypy may depend on orjson by default in the future.
+
 Types of empty collections
 --------------------------