I have a P100D and I'm certain it's the same for the P90D.
In order to get the absolute fastest acceleration time you need to allow battery to get to maximum operating temperature. This is true for Ludicrous and Ludicrous + mode. You can still accelerate at a ridiculous rate in either mode but to get the best time you need to get the battery at max operating temperature.
This usually takes 30-40mins when battery is at a resting temperature. It is kind of annoying but luckily it's still pretty ridiculous fast in Ludicrous mode regardless of battery temperature.