I would do them on your volume day. Speed is better developed when you are fully rested and can actually move the bar as fast as possible. If you do speed work after heavy work or rep work you won't be able to move the bar as explosively. I'd say do your speed sets then hit some extra volume or some type of variation afterwards. My bench days look like this:
Heavy Dumbbell Work
Max out on a bench variation (board press, floor press, etc)
Work up to a top set of 6 on a different bench variation (close-grip, wide-grip, etc)
So I'm getting extra volume on both days, but one day the main work is for max strength and one day it is for explosive strength. I'd recommend something similar.