When spawning a zombie (this may effect other mobs, however i have not seen them), you may spawn them with a predetermined name. If you do so, their movement is really fast.

This only happens when the command is used, and does not happen when you apply a name tag to a zombie. Their super speed is not completely consistent, occasionally their speed may return to normal for a few seconds when changing movement behaviour.

Command:
/summon zombie ~ ~ ~ <spawnEvent> Name
A name must be given to cause the glitch