I’ve also been looking for lightweight PDF reader, because sometimes I have opened few software manuals with 2k pages each, so I want software to be really fast.
If you’re not looking for any sophisticated functions but truly lightweight solution then I can reccomend you:
qpdfview
I think it’s as fast as pdf reader can be, but has very limited functions other than opening PDF’s 
EDIT:
Oh, it don’t support dark mode I think, but with ctrl + i you can inverse colors, so basically all text goes white on black if you like it. I do. 