The QlikView Server does cache documents and user selections to give improve performance, the fact that the documents seem to be slowing down the longer they are used might indicate a resource problem on the server?
When you next get the slow down you could check the available memory on your server box incase that's the problem.
You wouldn't be able to give a user the ability to clear cache as only a restart of the QVS service on the server will do that.
If resources on the server are the problem you could think about adding more RAM, looking at the amount of data inside your documents and any performance changes you could make. Performance and optimising are a wide subjects so its best to try and narrow down the cause first.