I am getting LOW Fps, because of simple Physics. Help.

Some thoughts... Your boxes have a box collider on them, and that has size 1x1x1. Your boxes have a mass of 1. So, you have a one metre cube that weighs 1kg. They must be made out of some super aeronautical foam material that is mostly empty space. It's not realistic.

Also, a stack of boxes face on is about the worst case scenario for a physics engine. The physics engine has to constantly adjust the positions of each box to stop them penetrating each other. For a stack of boxes, if one box is moved very slightly up to stop it penetrating a box beneath it, it'll penetrate more deeply into the box above it. The amount of work needed to simulate this is huge. (I know conceptually you "just" need to move all the boxes up a little, but that's hard to implement in a physics solver.) You say you have "very simple physics" but in reality you have extremely complicated physics, just the everyday scenario is very simple to describe.

Unfortunately, I can't think of a simple way for you to solve this problem.

Ok i did a test with just the information you have provided. I did increase the gravity to y: -1000 to get a decent gravity feel but I had no lag. Here is my what i put together let me know if it works for you.

[1]: /storage/temp/10844-archive.zip

Here is a link with the project (deactivated non related scripts) you can drag the boxes with your cursor. You can adjust spawning in the Spawner object. https://www.dropbox.com/s/03dlm8mr1099y9l/blockubuilder.rar