Depends on several factors. Some are classified, but some are pretty easy to figure out based on the information you supply on the application. Any of the following might slow down the background checks (as CBP might need extra time to decide whether you are an acceptable risk):
- any previous arrests, for anything, anytime, anywhere
- any previous immigration/customs-related violations, anytime, anywhere
- extensive international travel within the last 7 years
- multiple relocations within the last 7 years (i.e. complicated address history)
- multiple different employers within the last 7 years
....those kinds of things.