In my experience on both Xbox One and PS4, the game is perfectly playable. Yes, the game takes a noticeable performance hit in the bog and when it rains. Its hardly unplayable though, just not as fluid as preferred.
But here are a couple of things I would love to see fixed, because (for me) they really annoy me like no other.
1. Improve the godawful crafting/inventory system management. That system so desperately needs a way to permanently hide items that you have no intention of crafting. The filters they have are nice, but they're hardly adequate. Its especially annoying later in the game when you're trying to upgrade your gear with rarer items. I would also add a separate tab for oils, potions and decoction AND add a button so that you can quickly switch between tabs.
2. Enable a method that forces Geralt out of combat mode. There is nothing I hate than being stuck in combat mode in where I am essentially trapped, because I can't jump. I cannot begin to describe how annoying it is. Maybe make it where you can press R3+L3 and you immediately are out of it. It may make you vulnerable, but at least it gives me a chance to get out of a tight spot.
3. Fix the swimming. Getting out of the water should not be such a pain in the ass.