Unfortunately, the forums are not really set up to diagnose these kinds of issues. I'd suggest you call Esri Technical Support.
However, there are various web debugging tools, like fiddler, firebug that let you see where slowness is occurring. You might look into these if you haven't already.
Another thing to rule out is network traffic. Is the slowness occurring on the network even before you get to our servers? You can use tracert to get some indication of this.
Open a command window (CMD) on your computer.
Type in: tracert services.arcgisonline.com
This will display the time in milliseconds it takes to just get to our server (not download tiles or anything) and display the various hops you go through from your computer to the internet to our server.
Run this command when things are performing well and run it again when things aren't. If you see a difference in the numbers that display, then it's the network that's slow.
If the numbers don't change, then it's possibly our services. We'd need to do more to troubleshoot. In this case, you should call technical support.
Hope this helps,
Mike
