Interestingly, I just checked one of our websites with 2000+ students (from our database of 1.5million user accounts, Moodle 4.0.x ... but see below) and the form loaded quickly. So, I wonder why it is slow with your Database?
Is your database set to log slow queries? (This is generally a good thing to enable.) That will get you the full SQL as seen by the database, and then you can inspect it, and try EXPLAINing it.
Latest change in this area of code was MDL-71761.
Oh! and coming for Moodle 4.2 is MDL-71261 https://github.com/moodle/moodle/commit/ebeaa4b3f2c85386b3f57ce90e255666daa1c831#diff-35ac490fcd10fbd8adc6451212c00ba8ae63242f02236ceb8b579773893dc9c3 - and I think we are already running that code in our 4.0 site, because we applied the change early. I think one of the reasons to do that was to fix the performance. So, if you can, try applying that change.
Is your database set to log slow queries? (This is generally a good thing to enable.) That will get you the full SQL as seen by the database, and then you can inspect it, and try EXPLAINing it.
Latest change in this area of code was MDL-71761.
Oh! and coming for Moodle 4.2 is MDL-71261 https://github.com/moodle/moodle/commit/ebeaa4b3f2c85386b3f57ce90e255666daa1c831#diff-35ac490fcd10fbd8adc6451212c00ba8ae63242f02236ceb8b579773893dc9c3 - and I think we are already running that code in our 4.0 site, because we applied the change early. I think one of the reasons to do that was to fix the performance. So, if you can, try applying that change.