I have used three methods:
Two Alka Seltzer tablets dissolved in a small container (produces massive amount of carbon dioxide, which first sedates the fish into total relaxation, and then kills it), ice water (tons of ice cubes in a bit of water), and decapitation.
The fastest method was decapitation.....instantaneous. Any fish size. I've done this several times with aquarium fish, and hundreds of times with "fishing" fish. It helps to first sprinkle the cutting board with a little salt. This prevents the fish from slipping, due to its slime coat. Use a razor-sharp knife, of course. A dull knife, as any boy scout knows, can glance off, and botch the job.
Alka Seltzer took about 20 seconds for fish to stop moving, following some slight quivering. Leave the fish in the solution for about 30 minutes to ensure it won't come to. The one fish that did not go under for me, was an Otocinclus; those are very tough fish. I completed the job with ice water, which made the oto quiver for a couple of seconds, and then became still. I left it in the ice water for about 30 minutes, to be sure.
BTW, clove oil and Alka Seltzer are used in the fisheries and aquaculture fields, to sedate fish for live sampling. Of course, smaller doses are used that are used for euthanasia, and the fish are also promptly revived in clear water, as soon as measurements are taken.