One thing I wonder is if the you used an ssd with the AC68u or if you replaced the time capsule drive with an 840 pro if you would get better performance for small files that are typical on most computers.
It would cost a fortune to buy a SSD large enough to use with Time Machine, unless the computer you are backing up has a very small drive itself. I guess I don't see the point anyway - who cares if your backup process and accessing your backup drive is faster? I guess I wouldn't mind, but it isn't something I'd pay big money for.
I find it interesting that Apple is working with Asus to make TM possible on the AC68u, particularly since I think it's still not officially supported even on the Airport Extreme (though it works fine - I am doing that myself). I imagine it's still a little less user-friendly than using an Apple router, but I am not speaking from experience on that.