This is just an experiment I want you to try.
Save your report to the file system. Pick up the report in Report Builder. Now deploy it to the server. Do you see the same thing? I am not suggesting this will solve your problem, but just take a look.
I have run into similar problem recently. I am suspecting the database drivers. Have a look here,
Make sure you look up the last link on the above article.
I have no idea why this worked, but it did. My execution time went from 15 minutes + to 15 seconds. I am not sure why but I am fine with the outcome. Thanks for this off the wall idea.