Take the file from poppler issue 1126. It's a file that doesn't look super complicated, a map of some caves.
With Poppler 21.08 it took 46 seconds to render in my relatively powerful i9-8950HK
Thanks to a patch from Thomas Freitag that time got reduced to 28 seconds by not recalculating again something we had already calculated and just copying it. Huge improvement! [This patch was developed prior to the filing of issue 1126, i guess Thomas had found similar issues on his own]
Then issue 1126 was created yesterday and it was clear that we were still super slow, mupdf/gs/firefox/chromium can render the file almost instantly, and we were at 28 seconds :/
So i had a look at the code and thought, why are we even copying the data if that data never changes? So I made a patch that brought rendering time down to 2.8 seconds :)
And then i had another look at the code and realized we were copying some more data that was never even used
After patching that we're standing at 1.5 seconds
The 3 combined patches have reduced the rendering time by more than 95%
P.S: This only applies to the Splash backend (used by Okular), the Cairo backend (used by Evince) is at 120 seconds, so someone should have a look because probably similar improvements can be made.