Mine does this too and just started out of nowhere a few months ago. I have a 970 and I doubt it's the problem but it could be some driver issue introduced in the last few months. Twitch, and some other video sites, will run essentially in slowmo. If I drag the tab over to my main monitor it's fine.
I will say that usually (but not always) the problem goes away if I use a different browser. Newer versions of Chrome might be the problem. I get this with baseball streaming sites too that stream at like 6000kbps and usually if I lower the bitrate it'll go away. Internet speed isn't the problem as it always goes away if watching on my primary monitor. It's a weird problem.