Roger that Steve ..
OSX EL Capitan 10.11.1 bug fix pack was released (originally 10.11.0) and is running fine on my MacBook Pro and my wife's iMac so far...
It sounds like you're being throttled for sure, but a test with nobody on your network other than your machine would validate that..
If you're not getting close to the performance bandwidth numbers they're charging you for, a call up their support food chain would be in order..
Let us know if there's anything further we can do to help you...
Regards,
Doug