For what ever reason TM throttles itself so as not to consume all of the bandwidth, i.e., the initial backup is exceedingly so, and any sort of restores are painfully slow.
This was one major reason why I opted for Carbon Copy Cloner, I use CCC for full system restores, and Time Machine for those oh crap moments that I need to easily restore a file or folder.
Theres not much you can do, other then grin and bear it 🙁