It's so easy to kill a fish very fast. Yes, it will wriggle for a while after, but you just hit it in the back of the head, or push your knife down so you cut the spine.
Then you can gut the fish...
That's the slower method I was talking about. The fastest way is to put the blade directly into it and sever the brain closer up. I have seen fish still opening and closing their mouths and wriggling as long as 15 minutes after lopping off their head at the spine. Especially catfish. If you hit the right spot though you can kill them pretty much instantly and can even leave the fish intact.