I was a bit worried after allthe reports of gamebreaking performance issues. Luckily, after playing all day, there really haven't been any big issues. Sure some animations for things happening in the background could have been animated in more than 3 frames (looking at the kids swinging their legs beneath the desk) and there was a funny t-pose for a split second before the screen went black to fade out of the cutscene. It could definately be better performance-wise, for sure. Not trying to downplay it.
But no real gamebreaking issues for me, no lag either so far. Overall I've been very happy, there are things that need to be improved for sure (like giving pokemon a speedboost while following), but there is a lot of good in the game as well.