As usual, Steve has the best answer.
I prefer to fire the deck gun myself in GWX 3.0. The crew gets too proficient at it, especially if the watch officer has a gunnery qualification.
I figure the gun crew would only be as good as I could train them to be, so with the increased reload times and compensating for rough seas, using myself as the yardstick for deck gun performance is the most realistic. Plus I like shooting it.
The stock 4-second reload time is a bit short, imo, but don't underestimate how quickly a well-trained gun crew can work. I've seen 155mm M198 howitzers reload and adjust fire in about ten seconds, and they have to swab the breech first and use bigger shells, and put powder bags in the breech, and place the primer. Admittedly, though, that is on land.
20 seconds seems too long to me, because there was a relay system for passing shells up, and even 120mm shells can be manhandled more quickly than that using a human chain.