If you are using the code in my December 31, 2011 post the variable "motorSpeed" controls the speed since it is used for the "delay" value between steps.
If your goal is to increase speed, remember that this stepper is geared down with a 1:64 ratio - so it will never spin very fast. That said, it can be improved with a different library - see my post of January 17, 2012, 10:14:20 AM - try the AccelStepper library mentioned there. The VIDEO at that post shows the AccelStepper in use - your stepper should turn at least this fast.
The AccelStepper Library has speed() and runSpeedToPosition() functions that may give you what you want. While the AccelStepper Library supports a maximum stepping speed to about 4kHz, the 28YJ-48 supports a maximum no-load step rate of 900pps (less that 1kHz).
The 28BYJ-48 has a step angle of 5.625°/64. Therefore, one full rotation requires 4,096 steps. At 900pps (with simultaneous port writes as mentioned in the post of Reply #19 on: January 14, 2012, 10:17:55 AM) the maximum rotation speed computes to 13rpm (unless my math was wrong) - (900/((360/5.625)*64))*60. However, look back in this thread and you'll see that sbright33 says that he can achieve 35rpm. I don't see how this is possible but that is what he says. Please note, however, that he is running the stepper with 12VDC and he has to de-power it when idle to avoid overheating, so he is driving the stepper very hard.
As to speed, remember that this stepper was designed to move the vents on air conditioners, that are popular in Asia, with a goal of high torque not high speed.
Good luck with your experiments.