Some musings on the "not fast enough" stuff:
I get it. We all want the perfect representation of the Mario Kart series, but realistically I don't think fast would have been possible, or at the very least preferable.
1) Space: Yes, Mario Kart is a large ride building, but big, fast rides take a lot of space, especially if you want to have some dark ride sections. And this ride has to fit in a lot of places where that space just is not available. Radiator Springs racers is gargantuan, and its "race" only takes like 30-40 seconds of ride time. For a full on Mario Kart race, you'd need even more space. Sure, they could have built a Gringotts-like ride, but that in itself already gets dinged for not being thrilling enough, so I think the same complaint would have happened in that situation.
2) Interactivity: I just don't see a way to meld a fast ride with the level of interactivity we see in this ride. Maybe you could have had a stationary or slowly moving shooting section, but IMO that would have led to a disjointed experience.
3) Character/Location representation: Say what you will, but this ride does a superb job taking you into and showcasing classic Mario kart locations and characters. You would have had a fraction of them in a dark ride coaster setup.
4) Family friendly experience: Nintendo is a family company, and probably wanted to flagship ride to appeal to the greatest number of people possible.
Overall, I can understand being disappointed in the lack of speed. But I think when it's laid out, they made the right decision that checks more boxes than a more thrilling ride would have been able to do.
I'm not fully sure that I "get" the AR hype, because there are so many other aspects that strike me as more impressive, both in the ride itself and the wider land.
I guess I'm just missing something.
Completely being enveloped in a MK race alongside MK characters, adding depth and effects, interplaying with the physical environments and screen media, with very good (if not perfect) head tracking, all while on a moving vehicle that spins is incredible. I just don't see how it's not possible to be impressed.