OK. In detail, here is the scenario...
I have Comcast internet service. I have subscribed to 54 mbps down and 10 mbps up, speeds. The Comcast router is a passthrough, hence no wireless or DHCP is enabled on it. Behind Comcast, I have Zyxel's Zywall USG 20W firewall/router. At present, this firewall/router is handing out IP address and active WLAN.
If I hard wire my laptop to a switch or Zyxel or Comcast router/firewall, I get about 56 mbps down and 12 mbps up. Superb speed, no problem. If I go wireless, my down speed drops to about 18 mbps and my up speed remains around 12 mbps. Not sure why my downstream just drops drastically and not affect my upstream on wireless.
FYI: I spoke to Zywall tech support and they were puzzled too. They had remotely logged into my Zywall router to look at the config and nothing seemed out of line. The firmware on Zywall is up-to-date. In fact, they had shipped me a new and replacement device thinking there might be some hardware problem but after swapping the device, I still encounter the same issue. My WLAN is set to channel 11 b/g/n with WPA2 for security.
Any suggestions on where and what to look for?