You can test by using a LAN cable instead of Wi-Fi. That should give you some clue as to where the issue might be. But, the lag is probably on the upload side of your connection - outside of your home. Not much that you can do about that.
You can also download inSSIDer from here:
Download Wireless Troubleshooting Software | MetaGeek
Then check to see if there are other wireless access points using the same frequency as yours.
There are some videos on that site that might interest you as well.