How to generate an occupancy grid of the scene?

Hello!

I have developed a scenario with a city and I want to automatically generate a map.

I have made a script that moves a gameobject in a flat with a certain altitude and resolution, and check if there are collisions. However, this script is toooo slow, since it only evaluates one point per execution of update function.

I have tried to address this problem by using the start function (the collisions are not detected), a thread (the gameobject cannot be moved), corroutine (the frequency is the same of the update function) and timer (the gameobject cannot be moved).

Do you know if there is another strategy to generate this occupancy grid?

Thanks in advance!

The idea is to get something like this:

But wasting a couple of hours instead of a complete day.