Quite to the contrary, I'd say this update is evidence of the inner loop being hyperoptimized!

MSVC's support for musttail is hot off the press:

> The [[msvc::musttail]] attribute, introduced in MSVC Build Tools version 14.50, is an experimental x64-only Microsoft-specific attribute that enforces tail-call optimization. [1]

MSVC Build Tools version 14.50 was released last month, and it only took a few weeks for the CPython crew to turn that around into a performance improvement.

[1] https://learn.microsoft.com/en-us/cpp/cpp/attributes?view=ms...