The gurus have already spoken on this for years now, no need to write new essays about it.
So here’s a quote from Steve Sounders’s “Even Faster Web Sites”:
Everything is a trade-off. When optimizing for performance, do not waste time trying to speed up code that does not consume a significant amount of time. Measure first. Back out of any optimization that does not provide an enjoyable benefit.
Code for quality. Clean, legible, well-organized code is easier to get right, easier to maintain, and easier to optimize. Avoid tricks except when they can be proven to substantially improve performance.