How old are we talking? If the CPU is >10 years old and/or some kind of ARM, it may not have hardware encryption acceleration, which means it’ll happen in software. I did that once, it was horrible. lscpu |grep -i aes
should probably tell you what you need to know.
You said you’re on Arch, you’ll want to go through their docs which are solid: https://wiki.archlinux.org/title/Dm-crypt/Encrypting_an_entire_system