Thanks again for all the good ideas and hints.

I took a look at the statements with the profiler, but I can see no difference between the statements. I also took a look at the system with Filemon and Procmon and I can see, that the query in the management studio only causes minor drive access and the query in the reporting services causes much more drive access. To me it still looks like doing an index scan vs. a full table access, but I am not sure about it. I will know try to switch of all indexes and look if the queries take the same time...

Regards

Oliver