Investigations into PYTHON-5211 have identified at least four optimizations to improve BSON encoding/decoding performance:

1. Skip _type_marker lookup for common built-in types.
2. Use PyDict_New() instead of PyObject_CallObject() where possible.
3. Use PyDict_SetItem() instead of PyObject_SetItem() where possible.
4. Use PyObject_Vectorcall() and its ilk instead of PyObject_CallFunctionObjArgs() and similar functions.

These four together improve performance by up to ~25% on our encoding and decoding BSON benchmarks consistently across Python versions.