Can you confirm this :
on a multi-core system, VS2010 is 'multi-threaded'. So in my example above, it starts project 1, project 2, project 3 and project 4 in the 'same time'. And in the output window, all lines beginning with 1> are about project 1, with 2> about project 2 and so on ...
When a project is built, another one is build with a new number as 5> and some times we can have a mix of output lines of different projects and the consequence of that is that it is very difficult to read and to find on which project an error occurs.
In the dialog "Tools -> Options -> Projects and Solutions -> Build and Run" you can change the number of parallel builds. The default is the [allegedly] number of processors in your system.