Lag denotes the event when an application fails to respond user inputs in a timely fashion and is considered one of the most common annoyances that impair online gaming experience. Despite years of effort devoted by game developers and network designers trying to overcome lags, gamers still suffer from this annoying phenomenon. It seems to many gamers that lag is an unavoidable part of online gaming and sometimes they just give up fighting it. In this paper, we tackle the lag problem by investigating the root cause of lags for gamers. We develop a software called Game Experience Monitor (GEM), which monitors the performance of gamers' computers and the quality of network paths during game play, and use the collected traces to correlate players' perceived experience to find out the common cause of lags. Our analysis reveals that, surprisingly, while it is a common belief that the instability of Internet paths is the major cause of lags, the overloading of players' computers in fact plays a more decisive role to lag. It is hoped that this counter-common-belief finding will motivate further research for providing a better infrastructure for gaming and other real-time interactive applications.