My Passport for Mac encrypting extremely long time (MacBook Air)

It's normal. Encrypting a 4 TB hard disk drive will take about a week, perhaps two weeks, but not more than that.

If you had chosen Mac OS Extended (Journaled, Encrypted) when initially formatting that disk, it would have been encrypted almost immediately. Subsequent to writing data, FV must encrypt its entire capacity. If you don't need the information on it, erase it and start over.

It can take days to encrypt such a large drive even with a USB3 connection. Just give it time and make sure the laptop stays on and is plugged into the charger.

I've seen it take 2 months tops/

Filevault state, open Terminal & paste these in…

    fdesetup status

    diskutil cs list | grep 'Conversion Progress'

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