Windows is telling me that I am getting roughly 78mbps while Speedtest.net tells me I am getting only around 3-5mbps. My ISP, Comcast, gives me about 100mbps speed when wired. Can someone explain this?
Speed is not the same as speed. Just like a car that can drive 200 mph won’t take you 200 miles in one hour (not even 2, if you are unlucky) while driving in a city.
I assume with the “speed” windows tells you about, you mean the entry in the Connection Status, which you can see by going to Network and Sharing Center and clicking on the connection. That speed is the current maximum speed to talk to the component you are directly connected to. In my experience that speed is extremely optimistic for WiFi connections. Actual transfer rates can easily be less by a factor 10. This is normal. I don’t actually know for certain why this happens, but I assume the reported speed is the maximum rate at which data is sent, and what we see is the effect of packet loss, and the TCP overhead to resolve packet loss – someone might come along and provide the actual reason with sources.
Another issue is that you could be connected to a component that isn’t your Comcast router. For example, if you are connected to a WiFi extender, and that extender is connected to the comcast router, the second connection might be the bottleneck.
I recommend copying a large file from one device within your home network to another one, to test actual speeds on different parts of your network.