SB-600 vs SB-900 - consistent flash

I have a couple speedlights I use when necessary but, honestly, I don't know much about them.  I know how to slave them and adjust flash power, and that's it.   I don't understand anything about what TTL means or any of the other intricacies of the flashes.   For the most part, they do what I need them to do, with the limited knowledge I have.    However, my friend's new photographer GF came over the other night with her SB-900 (I have an SB-600 and a junk Nissin) and was taking dozens of back-to-back shots with no recharge time for the flash.   My 600 can probably take 2-3 shots before it has to recharge.   Before I trade in my 600 for a 900, I just wanted to throw a line out to the people that know what they're talking about, just to be sure this isn't a setting I can change.    The girl with the 900 didn't know.    If the 600 can't do it, I have to have the 900.

I appreciate your help.