Want to try on the latest release? I can't reproduce with Release (29) or Nightly (32) on 10.9. If you can give some solid STR and a test case we can investigate further, but as it is now, this isn't very actionable from our end :-/
20 tabs, each with it's dev tools open. Zero lags! I'm in love.
We have a light and dark theme; you can toggle back and forth via the options panel (the little cog sprocket thing top left)