Is it faster to resize a Time Machine partition, or completely wipe it and back up again from scratch?
Unfortunately, I cannot give a catch-all answer. So my suggestion is to resize the partition, and, chances are that that may be faster. But if I'm being truthful, there are too many variables to determine which method would be objectively better at which sizes and such. Sometimes, these things will just take their time and no matter how you do it, you will be spending a significant amount of time waiting, so yes. Just choose a method and wait it through.