We used to use several different solutions with subsets of the projects, now though I use the solution load manager extension, http://visualstudiogallery.msdn.microsoft.com/66350dbe-ed01-... which brings down load times significantly. It basically lets you specify how projects should be loaded, so most can be loaded on demand. We have ~150 projects in total, ~10 million LOC, and VS handles it almost as well as a small solution.