If you want to speed up the initial time machine backup try entering the command below in a terminal window. To open up terminal just search for it using spotlight and start it.
Speed up time machine backup terminal command:
sudo sysctl debug.lowpri_throttle_enabled=0
Return to default when done (recommended):
sudo sysctl debug.lowpri_throttle_enabled=1
The reason for time machines initial backup being slow is that it defaults to a low priority mode. When you enter the command above it will increase the priority with the cost of using up more resources from your Mac. The benefit here is that the initial backup will go much faster, and I would certainly welcome that over some more power savings.
When you are done, just remember to return to the default mode, if not your machine may feel slow as other background tasks are affected by this change as well.