Yes and no. OsmAnd tends to have far more details to display — more data to display or filter out. I’m guessing that if you had a smaller rendering file (instruction for painting) along with a smaller obf (less data due to prefiltering), it’d be closer to comparable.

Edit: The link, which addresses slowness with GrapheneOS not experienced with stock ROM, seems to specifically address the (non-stock) FairPhone too.