That's always a hard one to answer, unless you're very specific about what you're looking for in an editor. Do you want to prioritize it being feature rich, easy to use, codec compatible, or just fast? Most editors lean to one or two, but not all of those traits.
It used to be the rule of thumb was the more feature rich the editor, the slower it encoded. For instance on the extreme end of that example, VirtualDub can be used for basic editing, and encodes way faster than say Sony Vegas Pro, and is free, but it's void of effects, and can bog down when playing very large files. It IS however open source vs retail, so it works with x264, which a lot of editors licensed to use h.264 don't.
Here's an article to get you started...