I've had some interesting experiences with Time Machine. While you're using the machine, it tends to take a back seat. The VERY first backup takes forEVER. (The Sandlot: FOR... EV... ERRR). But once you get past that initial backup, then you're good and they're usually fast. Any disk io will be preferred to time machine. It goes a lot faster when you're not using it too.
I bought a 4TB disk from Amazon that was defective from the moment I got it. Should have returned it but I stupidly decided to take it apart to see if I could put it in a different enclosure - the microUSB was the board, but the drive transferred slower than anything I've ever seen. Even though I bought my wife the same drive, her drive was perfect, but mine was slow as heck.
I have a 2TB WD External that overheated in a zipped container - ever since then, it transfers at USB 2 speeds and refuses to go faster. :/
My MacBook will backup to Time Machine when it's in a locked state - so I suggest setting that overnight, and let it do its thing.
Highly recommend Carbon Copy Cloner.