Sounds to me like you're short on CPU power on whatever you're trying to play. Several things you may try: make sure HVC and VP9 are disabled (since they need a LOT of CPU compared to h264), change the loop filter settings for h264, select a lower stream quality.

Youtube covering 3rd of the screen with its overlay is normal when the seek bar is up. Just move the mouse to make it disappear OR switch to Safari iOS/Youtube Mobile.