In XP, there was very much the idea of spending time to prepare for a sprint. Get requirements and preconditions worked out, clear out possible distractions.. Then you'd use a 1 - 2 week long sprint (aka actual crunch time) to knock out 80 - 90% of a feature with everyone on the team under high pressure and positive, constructive stress. And then you'd have some time after the sprint to work with low pressure, clean up technical debt, consolidate and build up for the next sprint.
And honestly, that's a pretty effective way to work if you plan and gear up for it.