[brokenmachine]

I would imagine that the physical inertia of the speaker would also be a factor.

Opamps can respond that quickly electrically, but you need more power the faster you want to move the speaker cone.

Say the mic is 5mm from the speaker. Sound travels at 340m/s. so 0.005/340=0.00001471s. So it takes 14.71 microseconds for the sound wave to travel that 5mm. That sounds awfully quick to have to reactively move a speaker cone accurately. Obviously speakers can move quickly enough to play at the correct frequencies but I don't know what their latency would be.

I'm sure there are other complications that I'm forgetting but it's definitely harder than you would think to cancel out sounds.