I've found that it is much better to move most of your code outside of App_Code and into a separate DLL which you build when needed, it plays much better with Visual Studio. In our case, the only thing we have in our App_Code directory is an ITask implementation (Scheduled Task) and a CMSLoaderAttribute to register that task, and that is only there because if it was in our DLL we would need to register the DLL in the GAC every time we have a new version.
Also, this way you don't have to build the entire website every time. We also did this to test custom code in an external library and kept Kentico as a web project, no need to build every time, just a simple application restart when swapping out a DLL. You may have reasons to build Kentico as an Application Project, but if it causes you more pain than it is worth, try something else!
And 7-10 minutes to build an application? And 20-25 before that? That is worse than most Enterprise Java/Maven projects I've been on, even on 5400RPM hard drives! Something is wrong IMO if it takes that long to build.