I believe LeagueofLegend is a game and it is taking maximum memory to slow down the system. If you minimize the game and perform certain operations (Browsing, searching, scanning) in your system, it will not work properly. Also, I would suggest you to perform a complete virus scan on the system. GPU temperature seems normal to me.
Also, since when are you facing this issue as you can perform system restore if it has started recently.
For more information on system restore click the link mentioned below:
NOTE: System restore will remove the applications installed after the restore point but does not delete any data.
Thanks & Regards