Use your mid-level processor/ lame windows power mgt/ whatever graphics card to your advantage... This is one of the cheaper tactics I've deployed. I also discovered this by accident on my inspiron gaming laptop. Specs:
Dell i7577-5241BLK-PUS Inspiron LED Display Gaming Laptop - 7th Gen Intel Core i5, GTX 1060 6GB Graphics, 8GB Memory, 128GB SSD + 1TB HDD

When I switch to battery power, there is a MAJOR slow-down in game speed for about 30 seconds. So what did I do...?

Step 1: Unplug the laptop before each time you fish.

Step 2: Enter key commands and catch fish at a super slow speed.

Step 3: Plug in after you finish fishing.

Step 4: Repeat.

It makes this sidequest laughable... Anyway, try it if you hate this sidequest as much as I do. It's a stain on an otherwise perfect (and my favorite) game.

***UPDATE (2/27)
***I used this same trick to get the Black Hole concurrence (gaining me the Mistwalker achievement) as well.