If your server is more than 3 years old or is perpetually very busy, there's a pretty good chance it qualifies as a "slow server." Slow servers obviously take a lot longer to accomplish anything on. Instead of taking two hours, it might take 3 or 4. However, if you choose the slow server option, we'll know in advance that it'll take a while. In such cases, we can queue up a few long running tasks and then go off and do something else.
You can bet that if you pre-pay the slow server option, we'll be a whole lot more generous when it comes time to billing for the job. It could take 4 or 5 hours for your machine to do all the work, but we'll be content to bill for only 3.