As others have said:

Write it and optimise only if it needs it

Get your algorithms right first

Ninety percent of your code will be fast enough - only certain blocks may need tweeking.

Case in point - on a recent project with over 1/2Mbyte of script and about 400 'instances' two 'instances' ran far too slowly. Most 'instances' ran in under 10 seconds while these two required 60 minutes, which was unacceptable.
An analysis (See comments on monitoring) showed that we had the following:

while(1)
{
   $thing = new thing;
   $thing->method(getc());
   print $thing->result();
   $thing->DESTROY;
}

All very well and good, but our class was heavily inherited and new executed no less than 60 lines of code, including multiple function calls. result went all the way up the tree to an AUTOLOAD handler. And all for a 10 line method

So, about 120 lines of code (and about 20 @INC function calls) to do 10 lines of code.

Time for some faster, locally optimised code AND VERY LOUD COMMENTS. Both in the local code and in the class which was being 'broken'. Nett result was a run time about 10 seconds. Which was acceptable for this project.
--
Butlerian Jihad now!

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