If you are using deltatime for your timing code it is very easy. Slap another ' * speedfactor#' variable on the end of your delta lines.
So for instance:
translateentity myentity,velx*delta*speedfactor,vely*delta*speedfactor,velz*delta*speedfactor
etc
So for instance:
translateentity myentity,velx*delta*speedfactor,vely*delta*speedfactor,velz*delta*speedfactor
etc