At this point in time there are 3 main reasons games don't get to run buttery smooth (as far as I'm aware):
1) GPU blacklisting: although this is becoming less serious over time as old devices die out, it's still not too hard to find old Windows systems or old Android phones where the GPU is buggy and prone to crashing, so the browser falls back to software rendering.
3) V-sync quality. This is a passing issue in Chrome (the Chromium team is aware of it and are working to fix it) and Firefox (which to me looks already fixed in Nightly), and is made worse by the fact the latest node-webkit is based on Chromium 38 which has the bug. This will be fixed in due course as subsequent Chromium and node-webkit updates come through.
There's nothing unsolvable here, we should see all three points improve through 2015 and beyond.