Part of it is just controls. BotW is nice and responsive, and Geralt feels incredibly sluggish in comparison because of all the presumably mocapped animations that have to play for every single little thing you do. Try running forward and turning around to run backwards in both games. Gerald has a hell of a time coping, but Link just does it.

Part of it though was definitely the design of the open world. The Witcher 3 just feels claustrophobic. You can't walk two steps without having to collect herbs or fight drowners or talk to NPCs. BotW spaces things out more, and I feel the pacing is improved by this.