Well, what is the network setup? Cable modem, router, wireless?
Things to try
1) Reboot all the networking equipment (cable modem, router, wireless APs). Retest. If speeds are still slow, go to 2.
2) Plug a computer directly into the LAN cable on the Cable Modem. You'll need to reboot the cable modem when you do this. Once you get hooked up with the internet. IF the speed tests are back to normal, then the problem is from your router / wireless side. If not, you'll need to call comcast because it could be that they are working on the cables in your area, or your cable modem might be bad.