> While I like faster startup time, I don't know 4% speed up is worth enough for this complexity.

There's no magic bullet that I know of to reduce startup time by a large amount.  So we're left with optimizing progressively.  For comparison, https://github.com/python/cpython/pull/3279 had an even smaller effect on startup time.

This change has the nice property that it's a genuine speed-up on a generic operation, and it reduces the cost of adding slots as well as the cost of having long inheritance chains.