From what I understand, it is not the hatches that have a delay, it is the pressure plate - or linked mechanisms in general.
If I remember and understand correctly, there is a 100-step delay between the activation of a lever/plate and the effect of that activation. 100 steps is nowhere near as long as it sounds, but it can be a pretty long time when dealing with traps, and particularly with magma.
I would make sure that only a certain amount of magma is ever allowed into the "loading chamber" - probably with a lever-activated floodgate. This way, you can better control the amount of magma to come spouting down on your enemies (after all, you wouldn't want to release too much magma into your corridors while you wait for that 100-step delay between the magma activating the plate and the floodgate closing).
You might also use some grates on the trap-level floor to catch and divert the run-off magma, but this would also mean you might not get enough magma on the plates to turn the flow off. Then again, why would you want the flow to turn off automatically? Your Dwarves will flee from the magma, and your enemies will find it quite the gauntlet. A manual disarm might be precisely the way to go.
But it is, after all, your deathtrap.