What many people are implying but not quite stating is this: even a small gain in performance is exceedingly important if it is likely to aggregate.

Imagine you have a process you run 100 times a day at 2 seconds each. If you manage a 5% improvement, you only save 10 seconds a day. Probably not worth it.

Now imagine the same script needs to run 45000 times in a day... whoops, there aren't 90000 seconds in a day, so you'll have to speed it up somehow. A 5% efficiency improvement, though, means you'll finish on time (23.75 hours of processing for 45000 transactions). Performance certainly mattered here.

The only remaining question, then, is whether you anticipate given code will run often enough that a small gain in performance will aggregate into big gains overall -- and don't forget to plan for the future!