The speed of a drill bits is determined by the rate which the outer edge of the tool rotates in relation to the material being cut. In general, the SFM is within a range based upon the workpiece material, its condition, hardness and depth of the hole. The deeper the hole, the greater tendency for more heat to be generated.
Speed reduction is often recommended to minimize the amount of heat. It is usually best to start drilling at a slow speed and then increase.