Using functions repeatedly instead of variables

The difference is going to be pretty minimal, I’d expect.

There is overhead to using the functions, which you could save by storing your output in a variable. (WordPress does a pretty good job at caching the queries though.)

And the variable is going to be more convenient as it is probably fewer characters than the function.

A drawback to using a variable is that code will not be able to hook into the functions if that code runs after you’ve saved the value to the variable, so that could potentially cause trouble.

I am going to answer “you probably don’t need to worry about performance but there are other things to think about”.