There's several factors that go into decrypting a drive:
- The CPU - how quickly it can "crunch" the numbers to encrypt/decrypt
- Whether or not your have a T2 chip
- Drive size - bigger means more things to process
- Type of drive - SSDs are fairly quick whereas mechanical drives are much, much slower.
- How it's connected - USB 2.0 (480Mb/s) versus USB 3.0 (5Gb/s) versus SATA II (3Gb/s) versus SATA III (6Gb/s). All these different speeds factor in how quickly the decryption process goes.
When I had to do DoD wipes of mechanical drives, many of them were only 250GB, it could take up to 48 hours and it was just a "3 pass of writing zeros".
So I left my iMac on sleep mode overnight,
When you put your Mac to sleep, it stops the processes. Don't put it to sleep while decrypting.