Dan,

For Kings, Some say fast, some say slow.

I always troll fast, as in 2-3.5mph for both coho and kings, and frequently change speeds to make the bait/offering rise and drop, work differently.

I also think it is better to cover more water, and depending on what you are fishing you want the speed to make your offering work the right way. Really fast for Plugs, like 4 mph. Hoochies ans spoons, 2-3.5 mph. Herring, 1-2 mph.