Are you using Wireless or an Ethernet connection ?
Lag can be the result of a low Signal to Noise Ratio Margin (SNRM).
That can be caused either by local interference when using wireless or by using a channel in the router which is also being used by a few others.
A lot of routers these days have an auto channel setting which can drop you onto the wrong channel, but can be manually set to a specific channel.
Download inSSIDer3 to see which channel your router is using and how many others are on the same one.
The values are shown as a minus value and are the Received Signal Strength Indicators (RSSI).
Forget the minus sign, but the lower the value the better the signal strength and you want to be on a channel where the nearest user is in at least the 70s and yours about 50 or less.
This can also apply to users on adjacent channels as overlap can also affect your signal strength.
http://www.techspot.com/downloads/5936-inssider.htmlWhen the right pane populates, click on Networks.