Every time I plan on doing a Time Machine backup on Yosemite or newer, I open Terminal & enter "sudo sysctl debug.lowprithrottleenabled=0" - this stops it from being a background operation so it usually finishes before a clone even if both backups start together.
That use of Terminal is active for the login session only: if you restart or log out, it has to be redone for the next session.
There is a more complex process to make the setting permanent, but I'll just stick with what I know & can manage easily.