I have a large solution, of some 320 projects, where even small changes to a single web form result in long build times to test/debug a small change. I suspect post-build file
I think at 320 projects you've long ago exceeded the limit of what Visual Studio was designed to do comfortably.
There's lots of good advice here but if you really must keep 320 projects you might want to ditch Visual Studio for building and design your own build process.
Like I said in my comment, on windows at least, my recommendation for a build tool is psake which is built on powershell (and therefore included on every modern windows machine), quite powerful, and lightweight enough to commit the entire thing to your source control.