Anthony Ferrara has a new post to his blog today looking at something that's constantly a focus of user-facing applications, PHP ones or otherwise - application optimization. He specifically discusses handling it in PHP and some things to keep an eye out for.
When it comes to optimization, there are two competing viewpoints in the PHP community. Some say that optimization should only ever be an after thought and to avoid premature optimization at all costs. Others will say that this is impractical, and you should make your application fast as you write it, since then you won't have to go back and clean it up to make it faster. While I can understand the viewpoints of both sides, I am firmly in the former category.
He talks about the "90/10 Rule" of development (most of the work is done by only a small part of the code) and what that critical 10% could be made of. He also touches on the "60/40 Rule", "instinctual optimization" and the emphasis people put on benchmarks for their tool or framework of choice.