I don't know if users have that problem, but when I adding more than 30+ cities to favorites the Windy speed is going down.
I mean it's stuck, it's have some lags. It's not working when I on/off the compatible graphics, it's just heavy load the windy in every operation system it could be.
It's feels like I using a game with WebGL than using a weather application.
Even with one tab in the browser it's makes my browser freezing a lot.
Test it by yourself.
I have 2 smartphones, one tablet and one laptop, all of them is freezing when I have more than 30+ favorites.