There's no simple answer here, macke.
The simplest way is to install an accelerator, which caches the byte code generated by a PHP file (PHP files are recompiled each time they are run, or for each browser request), among other things.
Aside from this, the best thing you can do to "optimize" your code, is to actually debug it. Follow a tutorial on getting xdebug working, and have a look at the stack traces that are being created. What functions / operations are taking the longest? Usually you'll find things that you are doing repetitively / wrong that can be cleaned up.