Although I agree with Larry's view, at the same time I continue to be horrified by some of the code I come across. There's no question premature optimization is bad, but I have to think so is code that blatantly disregards and abuses speed issues.
For some time I have been meaning to look at the start up process for our application. There was no particular reason for this - no one had been complaining. But speed of start up is one of the first things people encounter and first impressions can be important. And we had had some complaints and found some problems with opening the help being slow, and some of the code is similar.
The first thing I looked at was database queries since they often dominate speed issues. I turned on the query tracing during startup and to my horror, ended up with 1800 lines of query trace! At first I thought that was 1800 queries, but there are multiple lines per query so it was actually only about 600 queries. It's a testament to the speed of computers and networks these days, that this doesn't even result in a noticeable slowdown. Although I think we did have one customer complain that after restarting their server, when all the users tried to log back in all at once, it was slow. I can understand why! Of course, we discounted it, thinking that, of course it'll be slow.
I have various ideas for reducing these startup queries, but I was able to cut them in half within a few minutes. Most of these queries are reading the menu tree for the application. At the same time it is checking for any menu options that are "hidden" (e.g. not purchased by that customer). To do this, the code was calling a general purpose permissions function which did queries on the permission table. But in this case all we needed to know was whether an option was hidden, which depended only on some configuration data in the code. We had a more specific function that only checked the configuration so switching to call this (didn't even need to change the arguments!) instantly eliminated almost half (about 300) of the queries.
I don't give this as an example of someone "screwing up". We all make mistakes or overlook things. The lesson that I take from it is that it's not enough to write some code and have it appear to work. You have to assume that there are always hidden problems and that you need to constantly be on the lookout for them. And constantly fighting entropy by fixing and refactoring.
See also my previous post on Slow Code