So, I've mostly played Stardew casually until very recently (Last week/2weeks), and I've gotten pretty used to using my controller. Has anyone ever tested how much faster Keyboard and mouse are?

(I assume they'll be faster since that's what the game was built for, but stranger things have happened)

I dont think anyone's tested how much faster it is, but it is considerably faster. Menuing quickly with the controller is basically impossible, mouse allows you to reach tiles around your character without moving, move the bed/tv reliably, check for forage behind trees without walking behind them, and probably a lot of other things im not thinking of. Its been a very long time since I played with a controller. I remember there being a bit of a learning curve getting used to keyboard/mouse controls, but once I got used to it I felt that playing the game overall was easier.