Troubleshooting a speed problem generally boils down to figuring out where the bottleneck is.
A quick look at your line stats indicates you are connected through to us at 3008kbps downstream with rather nice noise margins, so you may want to try a different speed test (to see if it's choked between Sonic.net and the cnet site specifically) or try an alternate configuration on your LAN (to see if it's choked locally). A traceroute may be instructive.
I'm generally skeptical of the accuracy of web speed tests, but it's pretty rare for one to be off by 2/3rds.