BleepingComputer.com: Why does my computer only use a fraction of it's internet speed?

My computer opens web pages very slowly an lags when I'm typing. I had a couple downloads going, so I checked the "Networking" section in Command Prompt. I'm only using 3-5% of my total speed (supposedly 100MB/S). Why would the internet be lagging if I'm only using that small percent of internet speed? I'm not wireless and n.o other device is connected to the internet. Nobody is hacking on to my Wi-Fi connection (if they did, I probably would have to congratulate them- WEP encryption, the ESSID is hidden, and it is configured only to accept wireless connections to my laptop.)

SPECS
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Windows XP Service Pack 3
Firefox 3.04
ISP: Verizon (Fios)
Actionec Verizon Router
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Thanks!
Jake