Thanks, Demi. I checked that first thing; unfortunately MS is not all that forthcoming on that site; everything looks hunky dory right now, but when performing the Network Test on the box, there is sometimes extra info on what particular problems they're currently experiencing.
I'm pretty sure it's them. I've not messed with my network other than to reset the router (just in case), and my other wifi using items are all operating at normal speeds. The xbox has been slow to do anything whenever it's connected to the internet, but when it fails to sign in to XBL, the speed is normal.
It is likely them prepping stuff for a fall update, possibly doing a rolling upgrade through their own servers, and the downtime is having a ripple effect on everyone's access of the network.