25m from what i have noticed after raiding both formats just requires more time. Not necessarily because of encounter difficulty but because of all the logistics outside of the raid. Things like someone forgetting flasks and needing to hearth or that guy with the bad net connection that has to restart his router every attempt. If you can put the kibosh on most of this behavior then it settles down into being similar to 10m raiding with a bit less personal accountability. Though just because there is the opportunity for it, doesn't mean you should be letting your raiders off the hook for derping about.