Very slow *WIRED* speed on one of my routers

I have a cable ISP and I get around 25MBps download when I hard wire the modem to my laptop.

When I hard wire via my Linksys WRTP54G router/ATA, my download speed is in the 8MBps range.

When I hard wire my via my ASUS RT-N13U router, my download speed in the 25MBps range.

NOTE: We're talking about HARD WIRING here not wireless - although the results are similar for wireless.

Both routers are at the factory default settings so there's no QOS...

Without doing any type of in depth analysis, the obvious answer is to just use the ASUS router. However, I use the Linksys for VoIP so need to use it at minimum as an ATA device.

Does it make any sense that the speed appears so throttled when hard wired on the Linksys? If it looks like it's a pure unfixable hardware issue, I'll just buy another ATA device. (Unfortunately, if it's a firmware issue, there's not much I can do as I need to use it as an ATA.)

Thanks in advance for your help.