I’ve been a long-Time Apple Time Machine for many years and it’s saved me many times.

I back up to a Synology NAS drive configured with Time Machine support. Time Machine has always been fast and efficient until I upgraded to macOS El Capitan (10.11). During that time, backups could be measure in hours instead of minutes previously. I initially thought the slowdown had to do with a recent software change in my Synology NAS.

I spent a bit of time in the Synology forums trying to solve the problem with various suggestions from users. Nothing worked.

I upgrade to macOS Sierra (10.12) and High Sierra (10.13) when they came out, hoping something had changed that fixed the agonizingly slow backups. No luck.

Disabling Throttling

I recently started to try to find a solution again since my backups recently went up in size and the slowness was really noticeable. Estimated backup times measured in days. After a bit of digging I found this suggestion, entered in a Terminal window:

 sudo sysctl debug.lowpri_throttle_enabled=0 

It worked beautifully. A very large backup went from days to hours. Subsequent backups now take minutes instead of hours.

Note: you can re-enable throttling with a similar command:

 sudo sysctl debug.lowpri_throttle_enabled=1

I admit I don’t know the side effects of disabling throttling, so use at your own risk.

Making it Stick

If you want to have throttling turned off between Mac restarts, you have to do a bit more work.

Create a file under /Library/LaunchDaemons/fix-el-capitan-slow-time-machine-speed.plist with the contents:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Label</key>
    <string>fix-el-capitan-slow-time-machine-speed</string>
    <key>ProgramArguments</key>
    <array>
      <string>/usr/sbin/sysctl</string>
      <string>debug.lowpri_throttle_enabled=0</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
  </dict>
</plist>

Make it belong to root:

 sudo chown root /Library/LaunchDaemons/fix-el-capitan-slow-time-machine-speed.plist

Make it load at startup:

 sudo launchctl load /Library/LaunchDaemons/fix-el-capitan-slow-time-machine-speed.plist

Conclusion

I’ve also tested on macOS Mojave (10.14.1) and can confirm a nice speedup. Overall, everything works really well now. I can’t take credit for the solution as I did some searching and came across several places this was mentioned, including one from Apple. I just know that it works!