I believe the newest version incorporated some fixes from Asus that fixed jitter on the wired connections. Ping times to the router itself would vary by 10ms or so and now they are a rock solid 1mS on my RT-AC66. This may have only affected the AC66R though.
Welcome To SNBForums
SNBForums is a community for anyone who wants to learn about or discuss the latest in wireless routers, network storage and the ins and outs of building and maintaining a small network.
If you'd like to post a question, simply register and have at it!
While you're at it, please check out SmallNetBuilder for product reviews and our famous Router Charts, Ranker and plenty more!