There's a tricky balance between the mass of the powder charge, the burn rate of the particular powder, and the pressure generated.
A few things to keep in mind:
Regardless of the burn rate, most powders will make a pretty similar amount of gas for a given mass of powder.
Higher pressures increase the effective burn rate of all powders.
Bullet velocity is dependent not on peak pressure, but upon the area under the pressure curve. Pressure equals acceleration, but the duration of acceleration is just as important.
If you were to use a really fine powder, it burn rate would be increased of course. That increased burn rate would result in higher peak pressures, which would necessitate a smaller mass of powder to keep pressures low enough for safety. The smaller mass of powder would produce a smaller volume of gas. In essence the pressure curve would peak very quickly as all the powder burned, but it would taper off again just as quickly. You would get a very short pressure spike with less area under the curve than you would achieve with a slower burning powder.
If you did the opposite and used a coarser, slower powder; the peak pressure would drop. This would allow you to add more powder to bump up the peak pressure again. Pressure would build slightly slower than with the fast powder, but still fairly quickly. After peaking, the pressure curve would taper off more slowly due to the greater gas volume being produced by the larger mass of powder. The pressure curve would have more area under it and the bullet would end up going faster.
The highest velocities are usually seen with the powder that is just fast enough to reach the limit of safe peak pressure with a fully packed case.
Now, there are variations in the powder mass/gas volume ratios for different powders and the powder manufacturers are working on new powders with higher ratios (which is how some new rounds manage to achieve the velocity numbers that they do), but hopefully the above explanation gives some idea of why things work they way they do.