Whether you need isolation for firing the laser will be specified in the documentation for the laser controller.
Also consider wither Mach3 is creating the delay. There are spin up and spin down delays in Mach3. These are usually to allow the spindle to come up to speed and to slow down to stop.
If you look in the Config->Ports&Pins->Spindle Setup tab you will see the;
CW Delay Spin UP
CCW Delay Spin UP
CW Delay Spin Down
CW Delay Spin Down
Set all these to 0
Cheers,
Peter