Having slowdown issues with my PC

So I built my computer in February. (PC Info: imgur.com/a/vQm41) I have started to run into slowdowns when doing simple things like watching videos on YouTube or streaming from my console to PC. All my PC games play fine, but I cant do much else while playing them on my second screen. I try to stream to Twitch, but it will cause some games like League to lag bad if I do, even if the ping is 50ms, and fps shows 60. (Network tests: imgur.com/a/3xW40) I thought I had bought top of the line and wouldn't have to worry about anything. Probably made a stupid mistake when setting everything up. Sorry if this is kind of scattered. Any help is appreciated. Thanks. :)