Good god man you still have time.
Make sure every brush that is not sealing anything or protecting from leaks in your lvl is a func_detail.
Next vis portal it up, there's an example map and tutorial on the hl2 sdk page this will GREATLY improve how well it runs:
http://www.valve-erc.com/srcsdk/Levels/performance_and_visibility.html
Next make sure there are no brush's cutting into each other, it sshould all be flush.
Were you can use a model instead of a brush do it.
If the model dsnt have physics properties and you dnt wnt it to be a solid, i.e just for show then make it a prop_detail.
Make sure every face tha isnt show is nodraw
Snap as much of it to grid (may not help but possibly might)
take out as many physics props you can with out altering gamplay.
Make sure each model is assigned the the cheapest type it can be, look here for more details:
http://www.valve-erc.com/srcsdk/Levels/prop_types.html
YOU CAN DO IT
now go go go go
Sigs