This is a cross post from superuser.com since no one over there seems to know anything.
I recently replaced a dying D-Link router (DI-524) with a Cisco/Linksys RSV4000. I'm really happy with the device but it seems when my cable modem is plugged in to it I lose 1/3 of my download speed. When I plug the modem right into my desktop I get 30mb/s down and 5mb/s up, but going through the router I get 20mb/s down.
I had a problem like this at work when we switched to Comcast as their boost service didn't work with the http proxy on our Watchguard firewall. So I tried turning off every option in the router to see if that had an effect but that didn’t change anything. I never had this problem with the old D-Link.
Someone said to check my MTU and change it to 1400 from 1500 but that didn't do anything on either the router or the desktops.