There is nothing particularly wrong with Okular’s performance, sure it is not as fast as MuPDF, but that’s okay. Okular feeling slow is, I would argue, a problem is with its interface, not performance.
Summary
Using the mouse, apart from dragging the scroll bar, there is no way to tell Okular to scroll down with lightning speed, say, 20 pages.
If you use the hand tool, you have to click and drag, and drag, and drag, and drag, and you’ve moved your hand almost a metre, and you’ve only scrolled down 10 pages. Or you can turn the wheel, and keep turning it and turning it and turning it and getting maybe one page per second (if you have to flick through a 500 page book, that just won’t cut it).
Keyboard scrolling is not any better.
Depending on the zoom level, if you press the Page
Down key, that only scrolls a few lines down, not a whole page! You have to zoom out to get a decent speed, which is inconvenient.
If you use keyboard autoscroll, the top speed of scrolling is capped to several seconds per page. Besides, I prefer interfaces that are either keyboard only, or mouse only, so I can do it with one hand.
Left and right arrows are pretty fast, so you’re pretty much stuck with them, but that’s a rather mediocre way to flip pages. Also, they don’t work if you’re zoomed in, then they do a completely different thing: horizontal scrolling at snail’s speed.
Again, I’m nitpicking here, I still use Okular in spite of this.