Quite simply,
You can't go wrong: As long as you are breathing with the abdomen (lower lungs)--rather then the 'upper lungs' ...and...as long as the breath is even and smooth.
One of the really, REALLY good metaphors out there for this is, think a 'bellows' used to stimulate/keep a fire going. If you go to fast, and uneven the air gets forced out (tension?) too fast and might blow out the flames.
However, there is no "slowest" you can go that will not be beneficial in some way to the flames. You can go at a slow, even pace, or at a really slow pace... Either way, as long as "you're doing" something, the fire will be stimulated and will grow.
In the metaphor, your lungs/breathing are the bellows, the visualization/focus *on* the dan tien is the fire (focus on it being an activator of the point), the dan tien's energy itself represents a pot of water, --which by you smoothly and gently doing the above correctly, causes the water to boil (purify into Shen/Spirit) and rise up the channels. The energy when it reaches it's purified state, condenses (like steam or cloud turning into pure droplets) returns to the dan tien as 'purified water'
Here's a useful hint when doing, Exercise, Meditation, or just plain 'out there in the world' and you notice that you get distracted or tense.
Anytime this happens, it's almost always either one of two things. A, you've 'forgot' to keep your focus on the dan tien. B, you're breath has become out of synch, and you forgot to keep it smooth. Don't ever worry about this of course, because every time you 'catch yourself doing that' and return your attention to dan tien or smooth breathing, you train the mind to do it more and more automatically.
I cannot think of any exceptions to this. Next time (any time) you are experiencing; tension, worry, doubt, etc, etc, etc,... Take a moment to notice how you're breathing is.
~Z