$ glxinfo | grep rendering
direct rendering: Yes
In my opinion this is not hardware related. I have the same behavior on 2 different machines, one with NVIDIA the other with AMD Radeon and completely different drivers.
If I don't touch the fast-forward bar a video plays smoothly from one end to the other.
It is only when chaining videos or moving inside a video when I get this slugginess and in the end the UI freezes. Also PM never crashes.
Same hardware and youtube was working perfectly fine 1-2 months ago.