Router Throughput Issues

We have a small office network and recently became aware of some substantial Internet<->network slowdowns. I spent some time trying to isolate the issue and found the culprit to be our Cisco VPN Router (RVL200). When the 'firewall' option was enabled, external traffic was slowed by a factor of 10. Granted the RVL200 is a low-end business-class router, but a slowdown of 10x is beyond the pale.

Our Internet connection speeds are rated at 20/1.7 MB/s by our ISP. Here's my findings:

ISP: Cox, cable broadband rated at 20/1.7 (download/upload).
Broadband Cable Modem: Motorola Surfboard SB5100, firmware version 2.3.6.0
Primary router: Cisco/Linksys RVL200 VPN Router, firmware 1.1.10.1
Router: Linksys Wireless G Broadband Router WRT45GS v7, firmware 7.50.8
Router: Netgear Wireless-G Router WGR614 v10, firmware 1.0.2.4_39.0.39NA

Speed test: http://www.speakeasy.net/speedtest
My location: Las Vegas
Test location: Los Angeles
Methodology: Asus netbook computer, Windows XP SP3, physical wired connection to devices, nothing else connected. Tests run a couple of times to confirm consistency.

Test 1: Surfboard: 14.13/4.85
Test 2: RVL200 (firewalled): 2.46/2.79
Test 3: RVL200 (not firewalled): 20.04/5.32
Test 4: RVL200 (firewalled, other settings disabled): 2.35/2.88
Test 5: WRT54GS (firewalled): 20.39/4.92
Test 6: WGR614 (firewalled): 20.8/5.41

Conclusion: the RVL200 doesn't have a firewall, it has a *brick* wall. We've since moved to a Sonicwall appliance for our VPN, so the RVL200 is looking for a new home...