If the problem is ONLY with one specific report and others run fine - Are you using import mode or direct query mode? If it is direct query, and if you are using a query to retrieve data, I would run it against the database and if it works the same way against the database.
All of the reports that have a lot of data are slow to run the first time. But after running one of the reports, the others reports that has not run yet will still be slow. So it is not a state for the whole report server, each report has to be opened one seperatly in order to open fast.
To me it seems like when the report opens, its model is cached in report server, the next time someone opens the report it will open fast.
We need something like a warmup script that opens all the reports, or, preferably, if there is a function in report server that handles this. Can't be only us having this issue.
From your response, it is not clear if it is on report or all reports. As I mentioned before, reports in report server tend to run slow after a period of inactivity. Did you look at the article I shared?