EQCSS.js was originally added in the Cosora Theme to spark a new paradigm of UI development, but it seems to have taken a toll on performance, despite the modifications I made to my copy of it to reduce the weight of the thing.
It's not so much that the library is slow, as much as it is it's insistence to scan every CSS file and not subsets of them. Beyond rolling my own EQCSS, or modifying it beyond what I'm willing to maintain, it would seem that there is little which can really be done to improve it's performance.
And despite intending to use it more, use of it has actually held steady with a single element query, one query which has a major impact on improvement everywhere. That is not acceptable. I think it's time to see it for the failed experiment that it was, it was a nice idea, but it's time to cut back.
It's already absent in the older themes like Shadow and in the upcoming Nox Theme. This should increase perceived performance for the end-user on first load in particular by a fair bit and it should be doable in a span of a few days of so, in contrast, to the time it would take to get anything meaningful done with EQCSS in the realm of performance.