Does the fish struggle with less might if it's being brought in more slowly? My naive guess is that it doesn't struggle appreciably less if being reeled in 25% more slowly. In that case, minimizing temperature is primarily a matter of minimizing the struggle time.

The faster you get a fish to the boat, the more likely they are to survive when you release them. With strong enough line the fish can't turn away and so is just dragged in without expending much energy. It will thrash side to side a bit, but it can't do long exhausting sprints away from the boat. When fishing for tuna, recreational fishermen will use 100lb mono because people enjoy fighting the fish and the fish aren't released. Commercial fishing uses 500lb dyneema line, and in my experience you release 3 fish per kill due to size limits. You just crank them to the boat and if they are too small they will swim away like they never missed a beat.