What version of FEMAP are you using? I just built a model of similar size with about 7400 elements on the surface and a pressure load. It expands nearly instantly. Do you have multiple load cases, other loads, a large number of other elements, anything else "large" in the model.
The physical size and shape of the elements and surfaces really should not matter as far as the time to expand ... just the number of items.
I realize that it might not be possible, but if you can send the model... or even a piece of the model that takes a long time, it could help me track down the issue.