PowerConnect 2608 slows down internet connection

SPECS:

Netopia Cayman ADSL Small Biz Gateway with 4-port switch, 10/100-BTX (model: 3546-002).

DHCP and NAT enabled on gateway router.

SBC DSL internet connectivity, dynamic IP, guaranteed 1.5-3.0 Mbps.

Dell PowerConnect 2608 unmanaged switch, 8-port, 10/100/1000-BT.

Dell PowerEdge 1600SC Server, SBS2003, Gigabit NIC, connected to switch.

Three identical client computers, Optiplex GX280, WinXP, Gigabit NIC, connected to switch.

All Cat6 cabling, standard patch cords purchased from reliable vendor.

 

PROBLEM:

Internet connection appears to be slower than expected for computers plugged to switch.

 

TEST & RESULTS:

 

All computers are accessing internal network with gigabit connectivity, as expected (LEDs on switch indicate blinking green for 1000 Mbps speeds, and solid green for full-duplex mode).  Therefore, cables and NICS are fine.

 

When connected to the switch, speed test (2wire.com SPEED TEST link) on each computer for internet connection ranges from 500-900 Kbps, below expected range.  Test was performed with only one computer having IE opened for speed test at any given time, instead of simultaneous internet connection from all computers.  No streaming audio or video was running in the background that may influence bandwidth results.  Speed test was done from one computer to the next so that only minutes elapsed between tests, not hours.

 

Next, connected one client computer directly to router rather than to switch.  Speed test gave result of ~ 2.2 Mbps for internet connection.  Retested with similar results (~2.5 Mbps).  Immediately connected the client computer back to the switch, and tested speed: result was slower speed of ~700 Kbps.

 

Next, tested another client computer in same manner (connect directly to router, then connect back to switch) with similar results.

 

Next, tested Server in same manner, with similar results.

 

HELP:

Based on my tests, there appears to be a real slow-down of internet connection when plugged into the switch rather than directly to the router.  Clearly, when connected to the router, I can attain the guaranteed range of 1.5-3.0 Mbps.  Why should there be a marked slow-down in connection to the internet when going through the switch?  The cables and computer NICs etc are clearly working properly, just differently depending on the whether they're connected to the switch or to the router.  I also realize that there may be slow-down when all three users are surfing the 'net, but this is why I tried to isolate the problem by testing only one computer at a time for internet connectivity.  Does any have any ideas?