Ok, I had a look around. I would have preferred the .map file to get a better idea of the real problems, but here’s what Ihave for you:
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Your terrain is way more complex than it needs to be. You have used very small divisions, which can make for some very nice looking terrain, but you could easily halve the number of peices without affecting the feel of the terrain.
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You have a line of sight throughout the entire map (this is your real problem). It’s possible to stand on the beach, look through the doors, see through the sand barricade and on into the rear depot. Because of this, the engine is drawing 2/3 of your map the whole time. To fix this, I recopmmend two things…
First, at the beach doors, consider extending those grey bunkers behind the wall so that they cover the doors (make the doors routes into the interior of the bunkers instead of the courtyard).
Second, do a similar thing behind the sand barricade. It’s al arge entity and so (remembering that entities cannot block vis) allows a lot of both sides of the depot to be visible from a lot of angles. Try makingf one side of it blocked by some form of structure to help reduce visibility.
Damocles final thought for the day: 
You should add caulk walls under the terrain where it meets the middle building (you know, where the two slopes of terrain connect to the sides of the building with the sand barricade). Make the caulk walls as high as you can without them being visible through the terrain.
PS - try lowering your skybox. If you do /r_showtris 1 you can see that the sky gets divided into a lot of smaller triangles. You don’t need the sky to be very high (thank you tracemap inventor! ) so you could easily lower it and reduce the number of sky triangles being drawn.
