I'd posted previously about how large ships in gravity causes major performance decrease and an increase in physics calculations. It is my understanding that what may be happening is that the cumulative gravitational force effected on all of the ships blocks exceeds the maximum force that thrusters can provide as the ship is not able to accelerate off of the planet (even if it could, the 3fps that the game runs at would make it a slow process). I believe a possible solution would be to decrease the amount of gravity effected on ships in a planet's gravity and apply it only to the astronaut or ships below a certain mass. I'm wondering if this issue is also related to large object collision causing lag as well.
In any case, I'd be happy to provide any information if possible.
In any case, I'd be happy to provide any information if possible.