also installing an up to date mesa if you are using a AMD or Intel GPU - that’s where most of the graphics optimization work goes. Updating kernel is mainly if you need support for stuff rather than performance tuning, e.g soon the upcoming 6.7 kernel for overclocking/undervolting support for 7000 series AMD gpus and at some point in the future Xe driver for discrete intel gpus.

Wolfenstein Enemy territory if you want some proper old school fps fun - 100% free and linux native - community is pretty small though there are a few populated community servers and communities that organise competitive games https://www.etlegacy.com/ (It’s also on steam but the client on steam is a bit out of date)