The contrast is generally controlled by addition time, the speed by gelatin quantity.

Fast pptn gives higher contrast and slow pptn gives lower contrast (in general).

Low gelatin, 1 - 5% is used for higher speed situations, and over 5% is used for slower speed emulsions, all else being equal.

Any Ammonia containing salts or silver halide solvents present during pptn will throw all of this out the window.

PE