Question about Promise ata100 controller...

[Trader05]

Hello everybody....I currently have an Abit-b6r2 board w/ a promise/maxtor ata100 controller card for my Ibm 60gxp hd. I have my ide config as, Promise = Primary Master Ibm Drive, Mobo= Primary Master Toshiba 48x cdrom = Secondary Master - Plextor 16x Cdrw. So i felt like messing around with my configuration, so i put everything on the Promise card and boy did everything go slow! WHY? I went to burn a cd, i couldn't do anything else! When the cd was done, nero said Burnproof avoided 2 bufferrun errors, i never got that message before. So i put everything back the way it was and it's fine again.. The question is why did everything slow down?