@ra-san and @rickkae
(caveat: my info may be outdated so you should definitely get answers directly from Tesla, but I still try to be helpful on this board when I can since I get a lot of value from it for all things EV)
I think it has more to do with the relatively tall gearing of the single speed transmission. at 1:8.25 (or something like that), the car is overcoming a very tall gear at first so you get a little bit of a slow start before it starts raising hell.
I remember when we were going from the 2 speed setup to the 1 speed, current was boosted from 650 to 850, but the gear ratio was going from 1:14 in first to a single ratio of 1:8.25. So a lot more current was flowing but to a much taller gear. Just like riding a bike, if you start in a higher gear its a bit slower to start but then you can go faster.
At one point in the development of the DT1.5, I rode in a test car with the higher flowing PEM and the 2 speed transmission. So I asked JB to put it in 1st and floor it and holy *sugar*. You can imagine the result. Total burnout insanity. As the marketing guy I was immediately thinking when everything stabilized with the company we should offer a 2 or 3 speed DSG type transmission for a high-cost option, but it doesn't make any financial sense.
I'm not sure, but I'm willing to bet with stickier and wider rubber on the back and a 2 speed dual clutch setup, the Roadster could get close to 3 seconds 0-60, assuming they could have solved the quick shift problem that bedeviled the 2 speed setup in the first place. But that's an awful lot of engineering and cost for a very small benefit.
DJS