I havent had this issue before, but as of the last few days I have experienced some major stuttering whenever I press a key on my mouse.
I am using a Roccat Nyth, and any of those extra (mapable) buttons cause the game to lag for up to .5s, but this happens on each keypress.
I am unsure on why this happens, or what causes it. Even on games with native Linux support, having proton active causes the same issue.