I know you can do create_function(): https://raw.githubusercontent.com/stonebig/sqlite_bro/master/docs/sqlite_bro.GIF

But the performance is crappy at best because of string conversion in the API.

If you want SQlite because of its performance, recent version starts to do multi-threading, and/or fear to leave other langages a stupid sizeable performance advantage, then you may perhaps don't leave the situation as it is.

Didn't Victor spoke about the need to 2X CPython performance recently, for it to stay in the game ? 

Look at latest Intel cpu shooting for quadcore, Iphone 8 being 6 cores, latest stable nodejs gaining a big speed bump, ... etc ... etc... and the need of efficiency to keep the planet cool enough.