Once the number of projects in a solution comes up to thirty or more, most of the project related operations become really slow: setting the default project, building a project starting and stopping a debug session etc. There are no noticeable slowdowns with up to twenty projects in a solution though.

Although usually no solution with real projects should reach such numbers (I stumbled upon it by just adding sample projects to the same solution), it is something to have in mind when deciding how to group projects in solutions.