For those unfamiliar with the term, lag time is the offset difference between the actual “run-time” of the media player and when it actually processes through the projector and becomes an image on your screen. Although it is measured in the hundredths/thousandths of a second, slow lag times can be a big deal with home theater applications specifically for gaming. Typical lag times are not really an issue in the business and education world, at worst perhaps having a very slight syncing issue on a video with a sound track.
Let's say you are a businessman with your own projector. Use it for work, take it home for play. Here’s the breakdown; a lag time of around 35 milliseconds or better is considered very good, even for serious real time gaming, such as first person shooters (Call of Duty). 50 ms is considered about as much lag as a serious gamer would want to deal with, and some will demand faster. Understand, 33ms works out to about being two frames behind on a 60fps source game, or one frame behind on 30fps content.
To put it in perspective, most home theater projectors with advanced features run 50 to 125 ms, so are slow for gaming unless you turn off features such as frame interpolation and other dynamic features that "look ahead" at least one frame (dynamic irises in most cases).
The image shows how we measure lag times. the front timer is the laptops screen, the back one is the projected image. In this shot that's a 55ms difference, although with best setup, I recorded 35 ms.