Squid is meant to be used as a general proxy for things like caching to help your local network save bandwidth, etc. Mitmproxy is more of a pen testing/security tool. They’re very different things and not something you would be comparing benchmarks to try to choose one.

FWIW from experience MITMproxy eventually reaches a point on my 2019 MacBook Pro where the whole computer becomes unusable. I have to restart MITMproxy.

I don't think they are doing all the necessary GC when disposing of flows from the UI.