Honestly, the game is really fun, but only for first two weeks or even less. After that the progress is slow and you will be doing just daily quests. There two oposite things that blocks continues playing experience: 1. Time limit for stage completion. Even if I can survive and beat strong boss within 5 minutes it doesn't worth it, following your logic. 2. Stamina mechanics. You are slowing grinding process on purpose. As for me the game is not f2p friendly at the moment.