It's slow because all the UI elements in the new themes have alpha transparency and rounded corners. It increases the amount of work your browser has to do to paint it tremendously (though GPU acceleration will help).
Rounded corners? AFAICT the defining characteristic of the new UI is that every single visual element is a rectangle with a pale grey 1px border. It's all so flat, ugh.
I've tried several stylish options to no avail, I miss the old dark layout with high contrast buttons.
Even the dark theme has a bright white message pane for no reason.
It also runs very sluggishly compared to the old UI, not sure why.
Well this should give me the kick in the pants to get off gmail anyway.