Well, it could be really fast. Like, really really fast. Also, if it's thrown, Naruto isn't near it with it blasts the enemy. So he avoids the damage that it causes when it goes "pop" on the enemy. Plus, if he can throw it, I suspect he can also control it, like make it follow the enemy. Like Yamcha's technique.

I really don't think that that jutsu he doesn't want anyone to see is the rasengan though. I just don't see a reason for him to hide that training from a bunch of frogs.