FileVault Encryption in no time on MacBook Pro M3

It is that fast because your data was already encrypted before you turned on FileVault. See the information in Protect data on your Mac with FileVault - Apple Support including the following:

If you have a Mac with Apple silicon or an Apple T2 Security Chip, your data is encrypted automatically. Turning on FileVault provides an extra layer of security by keeping someone from decrypting or getting access to your data without entering your login password. If you use a Mac that doesn’t have Apple silicon or the T2 chip, you need to turn on FileVault to encrypt your data.

Also, I know little to nothing about Terminal commands but my M1 iMac which has FileVault turned on shows the same result for # diskutil apfs list | grep 'Encrypted'

but if I open Disk Utility it shows encrypted. See below:

You won't notice any difference until you try to boot into Recovery Mode or boot from an external drive. When you boot to either of these, you will be presented with a request to authenticate....this is actually asking to authenticate in order to unlock Filevault even though Apple doesn't make this clear at all. Otherwise you won't see any difference while using the Apple Silicon Mac.

Just make sure to have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data. There are a lot of new ways to permanently lose access to the data on your internal boot drive these days due to all the new hardware, software, and security changes. Backups have never been more important.