At least, it *might* be a performance regression.  Here are the two commits I tried:

after:  ef4ac967e2f3a9a18330cc6abe14adb4bc3d0465 (PR #11617, from this issue)
before:  463572c8beb59fd9d6850440af48a5c5f4c0c0c9 (the one right before)

After building each (a normal build, not debug), I ran the micro-benchmark Raymond referred to ("./python Tools/scripts/var_access_benchmark.py") multiple times.  There was enough variability between runs from the same commit that I'm uncertain at this point if there really is any performance regression.  For the most part the results between the two commits are really close.  Also, the results from the "after" commit fall within the variability I've seen between runs of the "before" commit.

I'm going to try with the "performance" suite (https://github.com/python/performance) to see if it shows any regression.