why are some frameworks slow?
for example how comes that zend frameworks are slower than code igniter? if you don´t use a lot of classes, just chose the classes of your needs, then it shouldn't get slow?
are you forced to use a lot of classes with zend framework or are they preloading a lot of classes automatically?
It depends how much they have taken care of the speed when writing the code of the framework. CodeIgniter has been written amazingly always keeping the speed in mind. Also big frameworks autoload a lot of nuts and bolts making the framework slower. However, if you are experienced with any of these frameworks, you can speed up the framework to a good deal by filtering out the stuff that you think is not necessary for your current project.
Zends pretty heavy on autoloading a lot of crap.
Does anyone have advice on what to stop it autoloading and how?
Zend_Loader_Autoloader automatically loads classes when they are requested, so there are no "unneeded classes". There are some tutorials on how to speed up Zend framework powered applications, ie. commenting out "require_once's" and using the PluginLoader include file cache explained here (along with some other methods) http://framework.zend.com/manual/en/performance.html