Pretty much all of the PDF readers I have tried will work for form filling, however I have some similar issues with all of them.

I mainly use Okular or Atril.

Issue 1 is when filling out multiple fields in a PDF, it becomes extremely slow, to the point of typing some text, and having to wait for 5-10 second for it to show up and I can continue.

Issue 2 is that both Okular and Atril will insert the text with a much larger font size and/or different font than the document. Even in cases where the fields have some pre-populated text, if I touch the field, the font changes. Sometimes the change is significant enough that the text is not readable, or makes surrounding elements not readable.

The best way I have found that works is to use FireFox. The form filling in that works fast and doesn't mess up the fonts, but the way FireFox handles saving PDFs is tedious. I can't just click ctrl+s to save, as it prompts me to choose a location to save at and makes me overwrite the original file every time, rather than just editing it in place.

Is there any PDF reader that people are aware of that does not have these issues? Or is this something that is weird with my setup?

I'm running Debian 12 with the KDE Plasma desktop environment