A fast encryption for external hard drive Is it possible to set up encryption such that backups are still
reasonably fast? I will accept a 25 to 50% degrade in performance,
but not a 10x blowup in time to completion.


I have an 4 TB external hard drive, and I would like to use Time Machine on it to backup my 1 TB MacBook Pro (with SSD). The backups will happen manually every couple of weeks or so. Let me stress that leaving the external disk plugged in at all times is not an option.

Apparently I have a few options:


Normal hard drive, click on "Use Encryption" when connected with Time
Machine. (configuration I had until now). Seems way too slow; also,
every time I connect it I have to wait for ages until the drive is
decrypted and I can actually use whatever is on disk. Way too slow.
Format the drive and use Mac OS Journaled Extended, Encrypted.
This will encrypt the whole drive, as I understand it. But will it be
faster than option 1)? How do the encryption of 1) and 2) work
together? I am quite confused at this point.
The third option is to format the drive, use MacOS Journaled Extended
(not encrypted) and just use time machine without encryption.


Backstory

The first time I connected it to TimeMachine, it asked me if I wanted to encrypt the data. I thought "Sure, why not" and agreed. I waited for ages the first time (as it had to back up tons of data). A couple of weeks later I come back to backup again; this time it was about 20 Gb of data. It again took ages, and most of the time it was stuck on encryption.

I left it on the whole night, and the morning it was still encrypting (probably something went wrong in this particular case though, as it was 56% at 23:00 and 57% at 09:00 in the morning). But anyway it takes several hours for the decryption process, then there's the actual backup, and then several hours again for the encryption process. Honestly, I cannot have backups taking a whole day. I need to plug it in, wait 3-4 hours, then go on with my life. If I can do this with encryption, good. Otherwise I will have to remove encryption.
Mac OS Journaled Extended, Encrypted

I have external Time Machine drives formatted as Mac OS Journaled Extended, Encrypted. The encryption is fast enough not to be significant.

With this set-up, I have not experienced any problems or unacceptable back-up times. I too disconnect drives between back-ups.

Formatting and Mounting

Format the external drive using an encrypted format. This will ensure the whole disk is encrypted. Time Machine thus is no longer involved in the encryption process.

On connecting the drive, macOS will ask for the drive's passphrase. You can choose to remember the passphrase in your Keychain.

Encrypting

If Time Machine is handling the encryption, you will see a Sparse Disk Image Bundle in your external drive. This disk image file will be named after your computer.

If FileVault is handling the encryption, you will see a folder called Backups.backupdb in your external drive. This folder will contain a folder with the name of your computer.

You want the second, FileVault to transparently handle the encryption.