Hey...
I downloaded your level and checked it out...
Can't run it with just the .wmp
(Would need a script, and it would work better if I had the missing Wads.)
I can build it though...
If you want to build in BSP mode... Or rather... not in simple mode...
Try this:
Select all your plates and go to "File/export/export as mdl file"
I suggest though that you turn on the "merge" toggle
Then export it...
Then add the model... And erase the original plates...
It will build in 1 sec in BSP mode....
I figure it is the Tesslation process in the build that's slowing you down...
If you waited longer, it would probably build...
Good Luck !