Possible Memory Leak (or something similar?)
I'm not sure what's going on, but when I first open the client and play a few games everything runs smoothly, but as the hours go by it just gets slower and slower to the point where there's almost a full second lag between my messages being typed and appearing on the screen, or clicking on a tab and the tab showing up. Closing and restarting the client seems to fix the issue temporarily.
I checked the league processes in task manager and none of them seemed to be using what I would consider an abnormal amount of memory for a 2d game client (I'm just assuming that anything >1.5GB is abnormal) so I'm not sure what's going on exactly.