I have read somewhere that this is due to fuel pressure, but not real sure.
It takes a little longer for the fuel pump to reach optimum pressure (for the S54), DME senses when the pressure is in the correct range then fires the ignition.
I see the same issues, but if I had just turned off the car, then restart within the five minutes or so (approx), then I don't see a lag on starting. If the above is correct, then it takes a finite amount of time for the fuel rail to depressurize.
To go along with this, one other aspect could be the DME not allowing the engine starting in a lean condition which will damage your cats....