Depends on a lot of things. Depends on how fast the computer running the program is, how many events you have in your map total, how many events you have in the current screen, how many events are actually running at any given time, how complicated your events are, how many scripts you have running at one point of time, how complicated your scripts are, how compatible your scripts and events are... it goes on quite a bit, I'm afraid.
Best advice is to try to find an anti-lag script, and do your best not to stuff in more events than you need. If it still lags, try to condense what you do have, have events share commands instead of making two events for two commands, watch your timing for when you're asking events to input the commands, etc.