SELECTED ANSWER
replied on August 2, 2017
I mean, the Quick Fields Agent has to launch Quick Fields and shut it down after each document process. There's a time cost involved and launching and shutting down applications. It's in milliseconds, but it does add up.
Quick Fields Agent has limits on the number of sessions running at a given time. Sessions whose scheduled time comes up and can't find an available slot to run are queued and run in order as a slot becomes available. Queued instances of the same session are compacted into one run when a slot become available (for ex, if a session schedule comes up at 10:37:00 and there isn't a slot available, it gets queued. The next run comes up at 11:37:15 and there still isn't a slot available, it won't queue a second run since it's the same session and it's already in the queue).
When the session runs, it picks up all the documents in the target folder, not individual ones.
If processing those documents takes longer than 15 seconds, the next run will get queued.