Hello, I seem to have the same problem, but I have an even older computer. I usually get 100+ FPS while playing League, and once I open OBS, the FPS drops to 50-60. Here is the log file:
https://gist.github.com/anonymous/1dc81f2998783fef9e75
I deleted the Game Capture source I had, and that bumped my FPS back up to 100+. Per the advice in this thread, I tried using Display Capture instead, and that did work somewhat as my FPS ranged from 60-100. However, I'd prefer not to show my entire desktop. But when I try to crop to the window of the League game, I'm not able to find the window, even when I put the game on windowed borderless and I check the option to show windows without empty names.
I guess capturing my entire desktop is a workable option, but please let me know if there's anything else I can do short of buying a new computer (would more RAM help?). Thanks!!