Network Speed Went from 500MB/s to 21MB/s, Won't Return to Normal Speed

I signed up for Comcast September 15 and had speeds up to 1GB until October 21, when my entire network slowed to 21-24MB/s.  Now, everything downloads at that speed and has beend doing so for days.  It's taking 51 minutes to download an iOS update right now, for example.  What is going on with my network connection, could you please unthrottle it if it is throttled?  I need some answers, will call tomorrow.