CS6 uses Live Preflight (introduced in CS4) You can show/hide the preflight panel from the Window > Output menu and enable or disable the preflight function from the panel flyout menu. Preflight is known to cause lag on underpowered systems.
Live Preflight is a far more powerful tool than the old version. You can set up profiles to flag or ignore many different types of errors (and of course it works in real time). It also will provide a link to where each error is found, making it easy to locate and correct.