Do you happen to have a slideshow desktop background? Sometimes when the images switch it can cause lag.
Other than that, make sure you have no other programs running in the background while playing. Antivirus can also cause lag if it is performing scans. Let it finish its scans if it is doing any, or temporarily disable it (at your own risk).
Also check to make sure all of your drivers are up to date.
If you haven't restarted your computer in 3 days, definitely do that.