Using an old PC as a firewall/router... internet connection slows to a crawl.

I was doing some testing with Kerio Control on an old PC with a 1.8Ghz Celeron CPU and 1GB RAM — I believe it's a Dell Dimension 2300. I put three new Intel Pro1000 NICs in it, one for LAN, one for WAN and one for a VPN tunnel.

After installing the software and running some speed tests, my internet download speed when from 50Mb/sec to 5Mb/sec. WTF?

My question: Is this PC really that whimpy, or should I be troubleshooting elsewhere?

Edit: Thanks for the input! Much appreciated. I'll do some more troubleshooting/testing.