Yeah, I don't use those pages much, so I haven't noticed any page loading rate change at all.... and it's nice to be able to turn this feature on or off as some pages have colors, like usgs, that don't work well, makes them look murky.
The only slowdown I've encountered recently is the latest updated Ubuntu.... yes, I could've waited for the long term support one, but..... it was there... I had time... it's mostly slowed down the startup... fan sometimes... not often....
but this Dark Mode is nicer.... nice find.