Too much?
CSS Pixel art often requires heavy use of box-shadow, which can impact performances significantly.
I had to run various operations, including removals, to keep the final CSS under 10kb.
It’s a bit paradoxical, as the idea is to only use a few pixels to render the draws.
Still, it’s definitely possible to optimize it with some work.
You have to purge unused CSS, ditch any framworks you may use for layout and responsive, and sometimes use GIFs instead.