I would tend to suspect someone is doing some network throttling . Give this a try, assign a different port number to the Slingbox and port fowarding rules. Perhaps a more common port like 80 (HTTP) or 443 (HTTPS). If that doesn't improve it, then try make sure it is forwarding both respective UDP and TCP ports to your to Slingbox's private IP address. Most people tend to just enable it for TCP but forget about UDP.
VDLS or VDSL?