I have landed 15 lb Stripers on 5 lb leader on my my fly rod in about 5 minutes. 20 minutes is a long time for a 5 lb fish.
Frankly, I doubt it was 20 minutes. I've used ultra-lights with 6 lb test river fishing out of a canoe -- and five minutes is a reasonable time frame for a 4.5-5 lb black bass...if you choose to play him, rather than horse him in.
Never caught a pacu, but I would suspect that they're stronger than most game fish and would be a load -- probably given to running or going to the bottom than coming to the surface.
Given an inexperienced fisherman with light tackle, bank fishing and without the ability to maneuver, I can see ten minutes...which could well seem like twenty.
And, if the guy wants to tell his grandkids it took thirty minutes to beach the beast, well, I'm not going to object...
I fish 2 pound line and sometimes hook some 5 to 10 pound buglemouth bass that take a long time to land. If you are not used to catching bigguns on light line, you will tend to be too careful, but once you get the feel for fighting the fish using the rod, you can cut the fighting time.