OpenSnitch author here ... although performances are not a big deal here because only conntrack packets are intercepted, the project will move to a C++ implementation once the current Python prototype/PoC will be complete and will prove that what I want to do is 100% doable on GNU/Linux :)
I would say that golang is good enough. But if you are interested , you could go for Rust. Multiple teams have built extremely high performance network manipulation tools in Rust - like linkerd-tcp .
Once I saw OpenSnitch I decided to write my own in Rust (mostly as a learning experience for myself)[1] -- it's still obviously WIP as it doesn't even filter packets yet! My intention is to make it have a remote API so that policy decisions aren't done in the daemon but are done in swappable clients.
