If you’re willing to re-install anyway, you could do us a big favor and actually first try to change the iterations of slot0 to something like 500000 or 250000 and report its effect on your boot times. 
Instead of doing a complete re-install, you could also try to convert your current system to one with an unencrypted boot partition.
Pretty straightforward if you happen to have 300-500MB of free space for the /boot partition. If not, you could try resizing the luks partition to free up some space. You can find a description on how to do this here. You’d just have to adjust for btrfs (Disclaimer: haven’t done this myself for btrfs yet).