When you reference another project in combination with "Copy local = true", the specified assembly is being copied.
This is real easy, but has a big disadvantage: the assembly is being copied! :p
The problem is that you get the same assembly multiple times, which is unnecessary. Every assembly has to be copied/loaded/parsed by the C# compiler that apparently doesn’t come with some sort of optimization for this scenario. This really slows down the compilation process!
The solution is to specifically set the output folder (menu Project Properties > Build > Output path) and reference the assemblies from here.
...
If I'd only known this on my last project, where we had 42 projects and compilation time exceeded half a minute
inspiration