You are correct about the initial first time build delay, and it is done for each separate project. After playing with Intero on one of my stack based projects, I took the background time effort to pre-build all of my projects while exercising. Then, there is no delay (until I start a new project).

Stack has changed my development workflow, in a very positive way, and I love Intero (at least in the first 24 hours of use).