Is it faster to resize a Time Machine partition, or completely wipe it and back up again from scratch? I have a ~500GB Time Machine partition I'd like to shrink slightly. My internal source partition is around 400GB.

Resizing the partition would be extremely slow, since Disk Utility has to check all the hard links Time Machine is using behind the scenes. On the other hand, simply wiping it and backing up again from scratch would also be extremely slow.

Is one or the other generally going to be faster? Is there some disk size tipping point at which it becomes more economical to switch strategies?

(I'm using an SSD internally and a spinning hard drive externally, connected over USB 3. Both the source and the destination are encrypted. Also, I have a second backup disk, so if I wipe and back up again I won't be temporarily unprotected.)
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.