I have a Cr-48, which should be substantially slower than those with the Samsung Series 5's, and I have no real issue with laggy scrolling. Biggest tip I can give is to disable extensions, I do that and the Chromebook flies. Then just slowly add back in the most necessary ones. This might solve your video issues too, as video works perfectly smooth for me. If you doubt this, just hop into guest mode and see how ridiculously fast such meager hardware moves when your own bloat isn't slowing it down.
On Google+ you should really scroll with the "j" and "k" keys, problem solved. :P