My connection is 25mbps I believe, but we seem to get 30 with Comcast/Xfinity. For a short bit after we got a new modem, we had issues with lag and stuttering. But it seems to have worked itself out.
Our wifi is from a Time Capsule router, instead of the provided cable-modem router. We never notice any lag, and my wife and i are both on the computer a lot. She streams a lot of Netflix, and I do quite a bit of network-heavy work. We have a webcam that is running constantly, I have multiple computers that do backups over the network every half hour.
Do you do stuff within the house that you notice issues on as well, or just internet stuff?
According to the OP, it is only when accessing the internet, doesn't seem to be a lot of computer to computer or computer to NAS type of device interaction.