@vatinas I have the same problem and it annoys me a lot recently.
Unfortunately I can't give you any solution, the only thing I can tell you is that I'm using Chrome and it appears to be the same problem - so I think Twitch itself lowers the qualtiy. I hope not, because I've checked every setting in Twitch and haven't found anything related to that.
So hopefully it is something different and someone knows a solution.

@vatinas I have the same problem and it annoys me a lot recently.
Unfortunately I can't give you any solution, the only thing I can tell you is that I'm using Chrome and it appears to be the same problem - so I think Twitch itself lowers the qualtiy. I hope not, because I've checked every setting in Twitch and haven't found anything related to that.
So hopefully it is something different and someone knows a solution.