Oh lol, this is a VERY common error which people make.
Now, I'm pretty sure that you set your frame rate to over 30? Am I right or am I right?
The lag is actually caused by the browser's inability to keep up with Flash Player's frame rate.
The Maximum a browser can handle I'd like to think is 30fps... You can do with 40, but it will put strain on the browser and thus it might still get occasional lags.
My advice is to reduce your movie's frame rate to around 30.