As a halfback it can be hard to manage the team sometimes, there are really three scenarios I can see where I've had to slow the game down before and I've seen Genia do it in all three scenarios as well.
1) The piggies are losing their shape and you have a secure ruck where you can reset the structures
2) The 10 is calling players into positions you don't agree with and as the man with you the ball you more or less overrule his calling (this actually happened a few times between Barnes/Genia in the Wales series)
3) You're making a mistake
Overall the decision comes down to choosing between pressing forward with the quick ball and risking losing shape and potentially ending up with an isolated runner etc, versus deliberately slowing the attack down and resetting the shape of the attack. It's an incredibly difficult decision to make as you generally have 1 phase or less worth of time to make it, and you also have to be thinking several phases ahead as the potential outcome down the road is what you are basing your decision on. It's basically the same as scanning the play from 10 and thinking ahead a few phases except you have far less time to scan and make a decision.
It is beneficial at times to slow the game down, bring the forwards in tight for a phase or two and then picking up the tempo at the first clean ruck to create some space on the fringes.
The fact of the matter is that quick ball relies on the entire team working in unison (support runners close and cleaning out
quickly and effectively, the halfback being present as soon as the ball comes out, and the attacking players being aligned and ready to go in time) and as a result is very difficult to pull off for more than a few phases in succession. It's simply necessary to slow down the play sometimes or your risk:reward ratio will start to tip out of your favor.