Well, anything over the network is going to have a speed limit, which is going to depend on the connections and equipment used within the network (NAS, router, etc). I would recommend at least 1 gigabit ethernet being used as much as possible, especially between the NAS and the server Media Center is running on. But even then there may be moments where Media Center is trying to load data from the NAS and it appears to "freeze" or lock up until it can finish doing so.
Commonly this is the cause of Media Center freezing or locking up or becoming unresponsive when using a NAS, a network bottleneck occurs and the NAS isn't able to send data fast enough so Media Center is waiting for the task to finish so it becomes unresponsive waiting for it to finish. Which if that happens you should never close Media Center, let it sit and finish its task and it should become responsive again.
There is more to it of course, like how the drives in the NAS are configured which are going to determine the maximum speed possible from the NAS to the Media Center server, and other things of course.