I hadn't noticed this until someone else brought it up, but same here. 4.5 year old Zephr model used to shut off almost instantly but now I can hear the fans spinning for a good 20 seconds after the green power light turns off. It's definitely from the update.
Same here, I have one of those old fats and it suddenly started taking an extra 10ish seconds to turn off. At first I was worried maybe my xbox is starting to die on me, but glad to see its the same for everyone.