Yeah, only paralysis does lower speed, while burn lowers attack, and poison, sleep and freezing don't lower any stats. They only have their other effects.
I think I heard somewhere that the speed drops to around 30%... (from 100% to 30% that is) But I can't be sure. I'll go check it somewhere.
EDIT: Beaten! :P
EDIT2: Checked Bulbapedia, the drop is to 25%. Burn reduces attack by half.