If you're slow off the floor, the weight is too heavy.

Nobody lifts a 1RM deadlift quickly.
They also change the lift dynamic because of the compromised start position, meaning you'll probably pull differently... which means carry-over might not be as awesome as you'd like.
I would always contend that getting better at deadlifting means doing more deadlifts in the way one would normally do deadlifts - or variants of. By variants, I mean pauses at various heights, different resistance (bands/chains) or (I know you want to do something else, but) speed work.
Just my two pence - you'd get better training outcomes from doing more speed work than deficit work. IMHO.