Hello Studach Joerg,
We have made some optimizations through the recent years, however nothing specifically to address such an issue as we have not encountered it.
Can you please update to the most recent version (even if trial) just to verify whether there is an improvement in the loading times? Additionally can you replace RadScriptManager with ScriptManager and test whether the overall loading time is faster or not? (Perhaps the cause of the issue does not depend on RSM's combining operations and on the overall processing capabilities of the server.)
the Telerik team