It would seem to me that the server hardware is not up to the task. Running heavy reports will affect the workstation where it is run on,but it should not affect other connected clients in the manner that you're reporting. Can you specify what kind of hardware and OS you're running?
Also can you expand on the reporting mechanism, are you collecting a lot of data from unstored calcs?
It might be as Wim is concluding. A hardware problem.
But it could also be due to bad database design (data architecture).
- How large data sets is involved in your sub-summary and grand summary's?
- Are the data behind the summaries locally stored or related?
- Are there some interesting calculations behind the fields used in the summaries.
There are methods to ensure that the data used in the summaries etc.etc. are stored (versus unstored). And this may very well be the issue.
The easy way is to make sure that the hardware and the configuration is OK (please see my question about your configuration in my answer to your backup question).