On Mobile (using Mull) the PDFs are completely pixelated and simply dont render more details. Even after zooming in it takes forever.
Meanwhile on Desktop (Librewolf, Firefox, … on Fedora) it seems a bit slow compared to Okular, but it is very fine.
muPDF viewer is great. It only shows the contents of the PDF file and nothing else.
Just tried it, works very well and fast but I think I prefer MJ PDF also because it runs on hardened Chromium so less RAM etc.
Also MJ PDF has a dark mode, inverting black/white on texts