Hi Julius,

you mention that you are using a "windows package"

what do you mean by this? - I hope you don't mean that you have installed Moodle using the windows packages available on download.moodle.org - this is not a good idea for production sites.
see: Complete_install_packages_for_Windows#Security_matters

if you are using the packages - you should rebuild!

If you're using IIS - look at using fastcgi and a php accellerator to improve performance

even better - if you can move away from using a windows server - that is probably the best option to improve performance.