I'm not usually the type of guy that likes to say "stiff", but yeah.
If anything it's not the stiffness that really bothers me, but rather how slow your movements are. I don't know how to be extremely "in depth" when I say this, but try using more joints to enhance your speed and movement whenever you have the chance. Consider not ghosting your own joints as well, as little as possible. I'm sure you'd much rather have your joints ghost through uke later on in your replays.
Can't wait to see you improve though, you seem like an interesting replay maker.