I’ve just installed Manjaro Gnome 20.2.1 (on a test laptop Lenovo X240). When booting it, after typing the passphrase, I have to wait around 45 seconds. This sound to me terribly long.
That is normal when you use a default Calamares install.
There is a couple of reasons
I don’t recall the exact number of round trips used by Calamares but I think it is a high number (200000 or more) - the higher the number the better resilience against brute-force attacks.
Grub does not supported LUKS2 - only LUKS1 and the combination creates the long boot time.
The solution is to do a manual install using systemd as boot loader and LUKS2 encryption or select manual partitioning in Calamares and add an unencrypted partition (1G ext2 formatted) mounted on /boot.
Maybe - I don’t know - I really cannot imagine how it should be possible - after all the disk is encrypted. That alone makes it difficult to alter the installation. Some have tried to boot a live USB and tried to alter the encryption strategy manually - but I have never heard of any one succeed.
But if you are a seasoned Linux user - search the Arch wiki on LUKS - see what you can find. I am quite convinced that you will end up using a lot more hours and get a lot more frustrated and in the end you will ask yourself - why didn’t I just reinstall the system …
You can find several guides - also on this forum - or the archive describing in details how to accomplish the task at hand.
It is possible to convert LUKS1 to LUKS2 info here for instance
and the bootloader can also be changed.
The info on that is likely also in the Arch wiki
… it may be quicker to reinstall than to gather the information and then to adapt the current setup
like @linux-aarhus said