Look at the visual.ini file in the files/levelsbank folder of the standalone build and edit these lines at the top.
visuals.shaderlevels.terrain=3
visuals.shaderlevels.entities=3
visuals.shaderlevels.vegetation=3
3 should be the lowest settings, and then the player can raise it up by pressing Esc in game and adjusting the sliders.
What is the poly count of your level? About how many objects are in it? Did you happen to remove physics for objects that the player or AI will never get to? (trees and plants and border stuff)
Have you light mapped this level?
There may be ways to get the performance up for you.
What are you machine specifications and graphics card?
Thanks.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.