The second site is the one sighted. I could have sited the third, but I think its velocity figure is suspect.
http://en.wikipedia.org/wiki/Sling_%28weapon%29
http://christiananswers.net/dictionary/sling.html
http://www.slinging.org/
And yes, longbows fired 400ms, as did x-bows, and surprisingly slings. The accuracy of the sling and longbow is rather more suspect at this range though. I think the sling was inferior, because it didn't penetrate metal armor well. My instinct (as someone who understands physics) is that the sling was likely as effective as described. A roman text describes excising a sling bullet. You'll see it sited on one of the websites.
Slinging was cultural. So, those cultures that used slings often practiced with them, and the male population at large was skilled and accurate. Longbows, hard to use, were trained from childhood to very young conscripts. By the time they reached adulthood, they were excellent troops all around (better than most any other non-elite troop), but expensive. X-bows were the great equalizer, because they where very easy to use.