Good day team. This issue might be related to MDL-75043, but maybe some details are different.

The key to replicate this issue, is that the pdf should weigh some 30MB or more (though I've seen this problem with files as small as 16MB). The file should contain many images, around 10k-200k images, which can be as simple as rectangles and circles, like 16 pages of floor plans, each page with a 1MB weight or thereabouts (sadgely I don't have one such file of my own to publish here, but it could be created with architecture design software, or maybe with something as simple as Inkscape, and then copy paste the SVGs many many times).

Steps to replicate

1. Log in as a teacher.
2. Create a course or go to a course that you manage.
3. Create an assignment.
4. Enroll a student in the course. You should be able to log in as this student.
5. Log out.
6. Now log in as the student from step 4, go to the assignment from step 3, and submit your PDF with many images on it.
7. Run `php admin/cli/adhoc_task.php --execute='\assignfeedback_editpdf\task\convert_submission'`
8. Wait for the conversion task from step 7 to finish.

Observed behavior

The task takes too much time to finish, from 6 hours to actual days.

Expected behavior

The task takes a similar amount of time as when converting PDFs of the same weight that don't contain images (bitmap or vector graphics). Or at least it takes less time to finish than it currently takes.