You can download Visual Studio 2015 Update from here.
1- Tuned the algorithm for releasing cached project information to one particular shape of solutions.
2-Some cached project information was simply retained for too long, regardless of the solution.
3-VS enabled high-impact features like full code scanning in all cases, rather than allowing users to select whether they wanted them on or not.
4-When VS would fire events aimed at multiple projects in a solution, VS wouldn’t properly batch them; it processed them one by one.
5-VS would sometimes promote metadata references to project-to-project (P2P) references for a better experience; however, for some customers (those with complex P2P reference chains, or with post-build steps that modify binaries), this was actually degrading performance.
All of these fixes are included in Update 3.
For more details you can read the original blog post on a Microsoft blog.