As far as I know, there isn't a specific setting you can change to affect the cores VS 2015 Shell uses.30-45 minutes is about 28-43 minutes longer to load than I have ever seen on any of the largest solutions I've worked with. The largest ones I have ever encountered opened in a couple minutes max (and those were atypically large). I suspect there is something else going on with your machine. Here is a link from Microsoft on how you might improve performance, but there seems likely something else going on with your system.
One trick I have found is to close any open items in your solution on shut down. Since the solution will try to open those again on startup, that might be the cause of your extreme load times. You can right-click on any of the tabs for any of the open automations and select "Close All" to close everything at once.
Our automations pages are large, we have projects over 200MB/project.
We discovered that if you open project without any .os pages, run the project from the studio, pause estimately when you know that the execution flow is reaching your desired page, the shell uses all the configured cores to open the page, maybe becouse the runtime is executed.
So i think a tweak is possible, but our implications are limited by time and effort pressure.