There are too many duplicate selectors in version 4.5.2. In my site I find 48 duplicate selectors. Any recalculation of css causes a browser delay.
Here is the most striking case. The css framework/ spectrum.css - triple selector for "sp-container"

.sp-container{position:absolute;top:0;left:0;display:inline-block;z-index:9999994;overflow:hidden}
.sp-container{border-radius:0;background-color:#fff;padding:0;border-radius:4px;color:#000;box-shadow:0 0 0 1px rgba(99,114,130,.16),0 8px 16px rgba(27,39,51,.08)}
.sp-container{padding-bottom:0}

Contradictory rules can also be seen here

padding: 0  vs padding-bottom:0

border-radius: 0; vs border-radius: 4px;

None of our own CSS selectors use the .sp prefix - that comes from a third-party library we use. But frankly, the possible overheard for the browser is so negligible that it isn't even worth worrying about,