Enough speed means that you can accurately and consistantly hit at what you are aiming at...

. That could be taken two ways though, either 1) It does not matter what speed you shoot at as long as you can hit what you are aiming at...or 2) You won't be shooting fast enough until you can hit everything you aim at.....
...I prefer the former....
On a more serious note, you are going to hear a variety of opinions on this in regard to trajectory, kinetic energy and momentum levels, shot placement, etc... But, what it all comes down to, in my opinion, is what I mentioned above.