cst simulation volumen

Hi to all,
I am simulating a conformal array antenna in CST. The model is vary large so it takes a long time. I would like to know if I can simplify my model.
For example:
The antenna has a surface (material = PEC) with a certain thickness. I would like to know if I can mesh only the surface rejecting the rest of the volume. In the case I could do that, can I employ the transiet solver. Or have I to simulate the structure with the frequency solver?
Thank you so much.

My 5 cents contribution : I remember in college I heard scientist cutting substrate in some parts and stacking them; for the same global shape and volume, the accuracy or the simulation time was improved.
Will worth the price to test it...