In terms of the report, you may want to look at whether or not caching and refreshing at certain intervals makes sense - that's often a good way to manage reports with large data sets to process.
In terms of the errors, you'd probably want to look into the Report Server log to get more details and post any and all error messages. If you have a lot of this, you probably want to go through the ExecutionLog view and just aggregate and analyze some of the information to see where and what to work on for performance. Sometimes things like creating a stored procedure instead of using embedded SQL can make a difference. But what to look at and what to work on depends on the statistics you can gather from the ExecutionLog.

Sue