Time Machine is a great tool for backing up your system, but sometimes it can be very slow, in fact by default Time Machine prevents itself from using to much of your processing power. You can turn this off by opening Terminal in /Applications/Utilities and running:
sudo sysctl debug.lowpri_throttle_enabled=0
This will prevent Time Machine from throttling itself. To turn on the throttling again, just run the same command with 1 instead of 0, like this:
sudo sysctl debug.lowpri_throttle_enabled=1