I'm on a different machine now, but I'll have to go back and verify what I did -- it sounds like I may have set it up incorrectly. My /boot is a LUKS1 volume,

I'm on a different machine now, but I'll have to go back and verify what I did -- it sounds like I may have set it up incorrectly. My /boot is a LUKS1 volume, and I thought that my GRUB boot passphrase for this volume was in the first slot, and that I was using the same iteration count as the root volume. My / is a LUKS2 volume with a different passphrase that I need to enter after the initrd has been loaded. Decrypting the root volume is fast, so I suspect I set this one up correctly. Once the root volume is decrypted, I have a separate key on the filesystem to re-decrypt /boot so that it can be mounted without re-entering the /boot passphrase. This part is also fast. Something must have gone wonky with the boot passphrase. Either a crazy iteration count or a key slot that forces GRUB to not test it first.