Problem: Slow Drupal 6.x on Windows with over 30 contrib modules enabled. On Mac OS X and Linux, Drupal runs very fast but when i try to run same site under windows, it makes me cry.
I've tried hundreds of solutions but finally zend server ce was the correct solution. I was using xampp 1.7.3, then downgraded to 1.7.1 because of php 5.3 incompatible modules (such as imagecache). All problems fixed but performance. I prefer a portable server because when i have to reinstall windows, i don't want to backup my data, reinstall server software, reconfigure them and import/export databases. Zend sever is not portable but you won't need it to be portable, here is what you need to do: