I make my wimpy, low recoil reloads similar to SL1, using Clays. It's a quick, clean burning powder. It has a pretty tight charge window, so you have to pay close attention to the charge weight, but you should be doing that with every load anyway.
Lost sheep is right on the money with the pressure vs. velocity. It all goes back to calculus and the whole area under the curve thing they tell you all the time. Essentially the velocity you get equals(kind of) the area under the pressure curve. Lets say you make two loads using the same bullet but one with a fast powder, one with a slow. And lets say they both have the same peak pressure of 30,000 psi. The fast powder will quickly jump up to the peak pressure then quickly come back down. So there isn't a lot of area under the curve, therefore not much velocity. The slower powder will take a bit longer to reach peak pressure, but then it will stay up at a high pressure for quite a while before dropping down(or may not come down much at all until the bullet leaves the barrel). So you get much more area under the curve, therefore more velocity.
So why would you want to use a fast powder?? 1) you may not always want high velocity. 2) Fast powders tend to be more efficient. Yes, the slower burning powder will give you more velocity, but generally takes a lot more of a slow powder to gain that velocity increase. For example, when I reload my 460 S&W, I make my wimpy loads with 15 grains of Trail Boss. For my hot loads, I use 48 grains of Lil Gun. So over 3X the powder, but almost 1,000 fps in velocity gain(not the same bullet, but close in weight).