I set up an ancrypted LVM volume for full disk encryption with Leap 15. After the first password prompt by Grub it takes about 30 seconds from entering the password to actually booting. The second password prompt by the kernel is processed quickly. What is the hang up with Grub? Only software decryption? Running an X220 with a i5-2520m with is capable of hardware decryption and by no means a slow processor.
As far as I know, the slowness that you are seeing comes from your BIOS (or EFI firmware if this is a UEFI box).
I just booted up Leap 15 in a VM. From the time that I enter the encryption key, until grub displays the boot menu, is around 20 seconds.
When I do the same thing on a real machine, the delay is less than one second. I’m pretty sure that the difference is the slow BIOS emulation in the VM. I have also had real machines with slow BIOS.
Once the kernel is loaded, and the kernel disk drivers are in use, things go faster.