I used to backup on a USB flash drive, it was slow, and the drive became very hot in the middle of backup, so I bought a fast external SSD drive (2,800MB/s) which connects through Thunderbolt 3.

But a 60GB backup with Time Machine still takes about "1:30-2 Hours"...

Is this normal? I thought it would be a matter of minutes with this drive :(

macOS is Mojave if that matters, and I'm taking Encrypted backups on a Mac OS Extended (Case-sensitive, Journaled, Encrypted) formatted drive.

P.S. I found this command: sudo sysctl debug.lowpri_throttle_enabled=0 while googling for this matter, but it is for 2017, not sure if it is still useful and a good idea on macOS Mojave?


I am using a 2019 Macbook Air, my read speed is 650MB/s using this command: time dd if=/dev/zero bs=1024k of=tstfile count=1024

That's still 38GB per minute.

I also tried reformatting the drive to: CoreStorage Logical Volume • Mac OS Extended (Journaled, Encrypted) (not case-sensitive)


After running: sudo sysctl debug.lowpri\_throttle_enabled=0 it became much worse... even though I returned it to default enabled=1 , still 6 hours remaining. enter image description here

  • Is this the first TM backup on a new drive?
    – nohillside
    Jan 4, 2020 at 15:36
  • @nohillside Yeah, actually third time, but each time after a clean format of drive to test speed.
    – J. Doe
    Jan 4, 2020 at 15:38
  • The first time always takes long (on the Mac side, not the target side), so I don't see anything unusual here.
    – nohillside
    Jan 4, 2020 at 15:41
  • I’m assuming you have restarted after making the sysctl command?
    – bmike
    Jan 4, 2020 at 15:47
  • @bmike No, Is restart required?
    – J. Doe
    Jan 4, 2020 at 16:40

1 Answer 1


Time Machine backups are limited by three factors, the type of scan selected, the speed of the primary storage, the speed of the destination storage.

If your empirical evidence is increasing destination, then that’s not your bottleneck. (Sorry to restate the obvious)

I would look over the unified log file to see what’s actually happening and also not mess with any defaults (reset them if you changed anything) and use this tool to get some specific data:

It is a nice graphical wrapper and the author explains very well in their writing how it works, what some gotchas are and even how to read the system logs if you find you need different data than the Time Machine Mechanic utility exposes.

In the end, it’s probably more likely you need to exclude whatever data is taking a long time to scan or just live with the backup speeds, but I’d love a follow up question with some specifics if we want to get into if it’s bandwidth or iops that’s constraining your backups.

Also, case sensitive is an odd choice and not usual - do you have room to make a second partition and test the defaults journaled / encrypted HFS destination? The encryption isn’t going to make a difference in my experience either - so that’s a good thing to not test unless you’ve exhausted every other variable.

  • Tried reformatting to default non-case-sensitive Journaled, Encrypted, and also run that command, but its kind of stuck now, shows 6 hours remaining...
    – J. Doe
    Jan 4, 2020 at 15:37
  • @J.Doe The estimates are notoriously bad the first few days / intervals. What if you set Time Machine to not back up most of the disk and got some small tests of user folders - maybe pictures and music and avoided all the small system files? There are ways to peek into the details and tell what happened afterwards if you’re very curious.
    – bmike
    Jan 4, 2020 at 15:44

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