I was actually wondering the same thing, my reasoning is as follows (be it right or wrong is another issue): Hypokalemia, is a condition of low potassium in the blood, while several mechanisms may contribute to this from a electrochemical standpoint (loss of GI and kidney regulation), one mechanism is transcellular shifting of potassium ions from extracellular space (serum) into intracellular sapce. This increase in intracellular concentration would shift the normal equilibrium point (inside to outside) in the reverse, so on the phase 4 slope (where the depolarizing effects of the NA and Ca channels contibute to spontaneous depolarization of the SA Node) the normal outward flow of K would be reversed and therefore with another source of positive ion influx, cause the slope to increase (i.e depolarize quicker).
Alternatively, like a previous poster replied, if the loss of serum K is due to renal disease, there will be less of gradient estabished between extracellular and intracelluar K concentrations, this in effect will decrease the repolarizing effect of K ions in opposition to the depolarizing curents of NA and Ca in phase 4. With less of a negative influence ( by way of decreased loss of positive ions), the cell will depolarize quicker.
