This is one of those rare times I get to use that famous developer phrase, "That's a feature not a bug," and really mean it!
Previews are great. And a lot of people love them. But saving previews can be very very very slow -- especially for complex projects. This is why both Stacks and RapidWeaver now come with options to disable previews.
We decided to turn it off by default (at least for a while). We wanted to people to experience how fast Stacks 3 can be and then make the choice to enable that feature with open eyes.