This subject once came up on the CompuServe photo forum where Ctein is a longtime member. He offered an observation that I hope I'm retelling correctly.

If I'm getting this straight, his testing showed that an exposure time of at least 10 seconds was needed for a stable latent image. Anything less would make for an unstable latent image.

Hence, preflashing paper could not be done in batches and held for days or weeks, possibly not even for hours or minutes.

Again, blame me for errors or omissions in this retelling. That discussion occurred several years ago and, unfortunately, CompuServe threads are not archived indefinitely so there's no way to go back and verify my recollection.