Just walk away from the boss, and keep everything at fast mobspeed. Because the boss is constantly summoning new spawns, the boss just sits there, And the spawns walk to you. I do this all the time with laiden or Yoshi (when it gave xp) You don't need to change their speeds.
but that being said, you may need to change the amount of spwns because if you don't kill them fast enough then once the top number is reached(determined by the script) and you haven't killed them faster then they are spawned, then he will walk towards you. (usually doesn't matter tho as I always kill the spawns fast enough) (Wait, this might be what you are talking about eh? The spawn count being reached then the boss moves just as fast as you do?)