There's nothing wrong with your computer that you're getting that FPS, it's the connection that you have to the server. If a lot of people are using the internet at your house at the time, you'll get stuffty FPS.
I get 60fps even when swarmed by 20 zombies. :3wat
Okay. Here.If you have 4 people in your house. And their all using the internet at the same time. You're connection will be worse, thus leading to a worse connection to the server, which in actuality leads to a worse FPS. I don't know if I can dumb it down anymore.
floppy
I have 2 terrabytes (4k Gigabytes) on my harddrive. Try combining hard drives into one floppy and you get a stuffload space. It's expensive, but it's worth it.