> I am 100% sure you, as a manufacturer, could develop a very lightweight shim over a linux or BSD kernel that has significantly better performance than Android does

I’m not sure why this would be true, unless you’re willing to cut back significantly on features. Google already spends quite a bit of effort on performance improvements, and most manufacturers aren’t exactly known for being able to produce high-quality software.

There have been a couple projects that have tried to make phones based on pure Linux (Librem, Pinephone). Their performance is limited a lot by not having good CPU options, but it doesn’t seem like they’ve been able to achieve much performance improvement on the software side either.