Oh Im sorry if I wasnt clear. English is not my first language, so my excuses if I got it wrong or overly complex.
So I just try again, simple and easy, so everybody understands it right.
Build Project -> Builds the game projects changed files.
Rebuild Project (Rebuild Selection keyboard command as Rama suggested) -> Rebuilds the Engine, Editor and Game, just all of it. The difference for me is far more then 3 seconds (or 300, whatever that point means, thousands separators, decimal point and number grouping are not the same everywhere).
Thats with any version including 4.5 Preview, and thats the unwished behaviour some of us dont want. What inside askes for as far as I understand, and what I would find very convenient, would be that if you rebuild the game project, that it cleans and build just that project, and not anything else.
Might be helping to deselect the engine targets from the solution configuration to achieve that but Im not going to start another rebuild just now.
I hope this is easy to understand and clear. Maybe I did misunderstand something and beeing getting off topic, but I dont think I am.