Pre-clearance posts the time it opens for a particular flight, so if you get there too early, you'll be waiting in the duty free shopping area. Not sure how they enforce this, your boarding pass is scanned upon entering the area, but there are plenty of signs telling you to stay put until after the appropriate time. For my flight, that time was around 2 hours 15 minutes before departure. For others, it was a bit earlier.

On my last trip a couple of weeks ago, there was zero wait at security via fast track with my J boarding pass, and zero wait at pre-clearance, but that was for a bit later flight, getting to the airport around 8:30am,

Had plenty of time for two pints of Guinness draught in 51st & Green.