I have Linux installed with everything under
/, which is on LUKS. The problem is, after entering the password, I have to wait for about 10 seconds before the GRUB menu shows up.
Test the decryption time. Note that under linux the decryption is fast but since grub uses a shitty implement, it will be much slower.
Show iterations count
MK iterations: 125547
Make sure the key slot with the lower iteration is in the top slot.