Thanks to
@7oo1er who pointed me in the right direction, I found what was causing the
stuttering and
fixed it !!!!!
I've just made 2 races at 16 laps, with 16 karts in track, smooth as butter !!!!!!!
There was huge a log.txt generated in "Documents\Assetto Corsa\logs" after each race with the karts, it could took several GB.
You don't need to remove it, each race Assetto overwrites it.