Is he using Apple hardware? I didn't notice. Then of course, that's the cause of the problem.
Yes, now I notice "OSX" above; it is not "OSes".
et al:
The scourge of using linux on Apple hardware . . . !!! Mr Miata has pointed out the "apple" problem before, but I have been running grub on this machine for years without the level of slowness in the processing that seems to have crept in recently . . . to an extent that is diffficult to provide the time for. I don't use grub to boot OSX, so nothing in OSX is connected back into grub . . . .
AFAIK, it is not grub, it is os-prober, which is run by part of the grub installation process to find out what other bootable partitions are there. And as I recall, some types of partitions drive os-prober bonkers. AFAIK, that includes some Apple partitions.
Of course, you can simply not run os-prober, ever again, and create manual entries in grub for whatever other bootable partitions you have, in /boot/grub2/custom.cfg.
@Carlos, et al: Thanks for that suggestion on going to the manual entries in grub idea. I think that Felix M. made that suggestion to me previously, but I have a lot going on in my life these days and making yet more time to go manual (editing with each kernel upgrade, etc) seemed to require more time and thought energy than I have to spare at the moment . . . . It may turn out to be "the solution" . . . but these days if I can just "move it around" to another OS that somehow seems easier . . . at the moment. I can face into the fact that "pilot error" is involved, but previously that error did not take up such huge swathes of time to . . . compensate. Seems like something has changed on the software side that has increased the magnitude of the "bonkerism" in regards to os-prober. I'll try to post back if and when adjustments have been made, that have made a difference . . . . Thanks for the bandwidth.