I personally havent been bothered by the slowness of the OS. But I would be lying or stupid if I said that the OS wasnt slow. I have noticed a slight speed up in certain parts of the OS while in other parts I havent noticed any change in loading speed.
Booting up games such as Nano Assault Neo still take the same exact time, going to Miiverse while game (Nano Assault) is suspended in background still takes about the same time, though i have noticed (even before the update) that the more I go back and forth from game to Miiverse, the quicker it loads, going from Miiverse to friendlist is quicker I've noticed as well as going from friendlist to Wii U menu/ WaraWara Plaza. Booting up the Wii U is pretty much the same speed. Anyway that is all that I have done with my Wii U after updating, I havent gotten the chance to try out the Web Browser after the update or Netflix.
So I think it wouldnt be crazy to guess that this update wasnt meant to address the loading speeds. Though Nintendo has said that they are aware of the slowness and are working on it.
I expect that the OS speed will improve slightly if not greatly through the next few firmware updates that I expect will happen over the next few months. If we are lucky we will get another firmware update before New Years.
The update took about 30-40 minutes to download, though its worthy to note that I was also letting two youtube videos buffer in 1080p. As well as the 3DS update. Then the update took about 10 minutes (I wasnt really keeping track of the install speed so I really shouldnt be talking) to install after the Wii U told me to restart the system.
Also just a heads up, the Fifa 13 demo is up on the Wii U's eshop (in America at least). Im downloading it now and it is a rather large install, more than 1000 mb so its more than 2 times larger than the Wii U firmware update.