[lmm]

4 seconds is a long time. I'm reminded of the SVN fans who say things like "I can commit in 2 seconds, that's plenty fast enough". Which it is, until you've experienced the alternative, and then you can't imagine going back.

Also, all that separation isn't free. Sure, I don't need to run all my unit tests every time I make a change - but if they're fast enough that I can, that's much less cognitive overhead than having to think about which tests are relevant and press the correct button.