The first versions of Flask that supported Python 3 absolutely tanked performance. Some memory k/v store esque APIs we had that spent a good chunk of their time processing headers / query params to know what keys to look up got something like 8x slower. I remember ripping all the logic out of one endpoint and finding out it was >1ms minimum round trip, something like 85% of the time spent switching pure ASCII back and forth in six. It's a shame, because I absolutely love Flask, but we couldn't tolerate that so new APIs ended up in other languages.

I can't imagine what it would do to Mercurial's performance to have picked the wrong migration library early on.