Comment # 11 on bug 1184069 from Klaus Mueller

I don't think a preferred key slot is the right way to go. It's rather a bad
workaround - by far not the solution. Here it takes 22s to get the key on a
Ryzen 7 1700X CPU. This is 21s more than it should take!

As there is a second key, you have to wait 44(!)s if I mistyped the passphrase.
Afterwards you have to reboot, because grub even doesn't support to provide the
passphrase (grub 2.06 / Leap 15.4) a second time ...

All together it seems to me that the partition / fs decryption is a very
unloved child of the grub maintainers to put it diplomatically.