I'm having the same problem, but in KB rather than GB.
I had to stop the backup because my MacBook was out of battery, after having backed up aprox 50 GB out of 400 GB. I stopped the backup from the Time Machine menu, and ejected correctly, before disconnecting the external HD. After charging the macbook I plugged the backup disk back in, and then Time Machine spends about an hour "Preparing Backup." Then it starts copying, but stays in the KB for hours, until I run out of battery... and then I repeat the process...
I've been over everything on http://pondini.org/TM/D2.html. I've also tried booting in to safe mode, clearing pram, verifying / repairing disk & permissions on both the macbook and the external disk. Nothing I've tried gets the backup back to a normal speed.
MacBook is from 2015 and software is up to date.
External hard drive is a Western Digital 1TB.
Any help would be greatly appreciated!