not gonna go into it too deep, but uh, client is so bad i doubt any work can be done without either insane amount of work or complete overhaul. its funny actually how the game is less demanding than the client even with low spec mode on.
thing you can do: if you play on windows, on a laptop, change your power plan to performance because by default it has JavaScript perf on high, or change your current power plan to such. basically league client is a skinned browser, so these types of stuff i can feel when i change them, and its rather drastic.
it might be a pain, but if you could refrain from using the browser if your machine is not the greatest workhorse the league client will appreciate that too.
other than that, the client also have resolution scaling issues, i used to run my screen on a resolution where i DON'T need windows scaling because it often breaks the client when the game ends and coming out from full screen, either that or use border-less/windowed mode to avoid it.
that is as if you are talking about the client, the pre-game interface. it honestly shouldn't have impact once you are in-game, and u can set it to close it self when game starts. (it doesn't fully close it self but kills foreground process, and reopen then when you finish the game).
not sure how the client affects the game whilst you are in game, maybe just uninstall and then delete c://riot games (assuming u on windows) and then reinstall.
if you are talking client as in the game itself, i have no idea and it really shouldn't break.
mean while the minimizing button hasn't worked for years on os x lmao riot billion dollar company making html5 hardcore C/GPU-rape animations like a hentai when the dad comes home to his 2 step daughters.
in short, praise adobe, fk html5, fk "web developers" and especially fk website trying to have fancy animations.