the art direction generally doesn't change once its been revealed to the public
You're actually kinda wrong. Here's a look at early builds/prototype graphics for Street Fighter 4 before they did a big art direction change for the final product
http://www.eventhubs.com/news/2014/dec/27/looking-back-prototype-and-pre-release-builds-street-fighter-4/
Now, as far as the gameplay speed goes, it looks as if the speed being slow is just an illusion. I mean it looks about the same speed as SF4 but the difference is the amount of frames of animation per move. You'll notice that each normal and special attack is extremely fluid, meaning that the developer (actually not capcom, i believe it's a company called DIMPS) is adding more frames to create that fluid movement. Also, if you watch slow down gameplay, you'll see something MKX isn't even doing, which is a different animation for the end of a normal when it is blocked. There are instances like a roundhouse kick in which a the kick connects and Ryu does a full 360 with his body and the force gets pushed through his opponent. Now when the attack is blocked, the kick stops on Chun Li's arms and his leg recoils back, stopping him from his rotation which is more realistic. MKX cannot do this because NRS subscribes to using the canned target combo animations all the time, so you can do a full target combo on a blocking opponent in MK without the animation changing.
I'm not saying this is good or bad, in fact I fucking love MKX and it gets me a little chubby just thinking about it, but I would like people to understand the differences in animation styles and gameplay which might make people think a game looks faster or slower than another game.
