I have a little bit by way of Haskell chops, and I'll venture that the performance of the Hack typechecker is a very big deal, and it is in a different breed than the turnaround time you get from snap or yesod (or Java).
Edit: I misread what you wrote. I thought you were saying there was something fundamentally different about the type checker, but more that reload experience is different then what you get with Yesod.