I believe the difference here would involve one of two possible issues. 1. There may be a difference in the firing mechanism. For phasers the basic set-up is a button that is pressed and whilst that button is being pressed the beam is shooting out. For blasters, it is simply that you pull the trigger and out shoots a blast. As for the slowness, really have no idea perhaps they also developed technology to change the behavior of energy, anyhow. In theory either mechanism could be altered to behave like the other except if a blaster is designed simply to blast a complete redesign would be neccesary. 2. The other possible issue would likely have to do with the power source. A phaser to work with it's steady beam would have to have a steady reliable power source, such as the crystals that their ships use or today's modern batteries. Where as the blaster is likely using some sort of capacitor just like our cameras. If that was the issue, simply changing the type of input with the right modifications should make the alteration of output possible. . Added later Just so you know, this answer was for his question before AB editted it. Mentioning phasers as well.

i would guess you mean shooting at slower cyclical rates. any interrupter could be incorporated into the device to allow for measured times between pulses. the speed of energy is about the same as the speed of light...i dont think you could slow that down much