Where does performance matter?

Performance matters.

Sometimes.

In some cases.

It's really easy to get distracted by focusing on code performance.

It's easy to spend far too much time debating how to write code that executes a few milliseconds faster.

How do you determine/decide/measure whether it's worth discussing/debating/changing some code if the time spent thinking about, discussing, and then changing that code takes much more time than will be saved by the slightly more performant code?

Obviously, this depends on the code, where it runs, for how, long and how often it runs.

Is it worth a couple of hours of developer's time considering (and possibly making) changes that may only save each user a couple of seconds over the entire time they spend using the software?

What are you optimizing for?

How do you ensure developers are spending time focusing on what matters?

The performance of small pieces of code can be easy to measure.
The real productivity of developers is much harder to measure.

How do you balance getting people to focus on the hard things (that may also be difficult to quantify) and easy things to review/discuss (that they're often drawn to, and can look important from the outside) but don't actually "move the needle" in terms of shipping value or making the code easier to work with?