I somewhat blame SivHD's 'custom LoL interface' how-to. I've seen a TON of people following his method of using a global source monitor capture and adding it to a scene multiple times and cropping it down to rearrange game elements, when it's just outright bad performance-wise.
But yeah. Stop using Monitor Capture. Use a Game Capture for the in-game (if you use fullscreen) and a Window Capture for the pvp.net client (menu, pick/ban, etc). If you use fullscreen borderless mode, use a separate window capture for the game client too.