Interesting AMA with Fraser Speirs on Twitter who got a chance to use iPad Pro at London event. Mostly positive comments, especially about the Pencil. Apple really needs to bring it to iPad Air. If it's as good as people are saying I'd buy it in a heartbeat.
https://twitter.com/fraserspeirs/with_replies
Regarding lag...
https://twitter.com/fraserspeirs/with_replies
The Pencil was extremely impressive. Certainly the best iPad stylus I have used by a country mile. Palm rejection near-perfect. #ipadpro
Regarding lag...
None at all that I could perceive. Very, very direct drawing and writing experience. #ipadpro