why is my game not running as fast as it could?

this is more just about my computer rather than just the game. I just built a new computer with 64 gigs of ram, an Intel i7 6 core processor, and a gtx 1080. When I get into a game of league I have about 140 fps but after moving around and playing for a bit is decreases to 60 fps. 60 fps isn't bad, But when I want to stream league and having obs and discord open decreases my frames to 20-15 fps. I know my computer can do much better than that. When I watch streamers like imaqtpie, tyler1, and Yasuo they are playing at a constant 140 fps and they obviously have obs open, as well as other stuff. I'm just wondering if anyone knows any computer settings that could fix this. I'm positive that my computer can do better.