Rebirths are straight up punishing. The bottleneck in this game isn't XP gain. It's training speed. The area after the Sunken Grotto is a giant leap in difficulty and requires hours spent AFK to progress, every single rebirth that is. With no way to speed up rebirths, each play-through gets longer and longer, with your progress in the previous rebirth just basically deleted, since rebirth points are almost meaningless in this game. Even the unlocks you get each rebirth don't seem all that meaningful or game changing since the bonuses are very small.