Objects not colliding with other colliders and falling slowly

Hello everyone!

For some reason, the physics of my objects break and so do the colliders once I add them via script or I assign the collider and rigidbody manually.

What happens is the cube sinks through another plane that has no rigidbody, but it does have a mesh collider. The cube has a rigidbody with all the default settings, and a box collider. Also, the cube falls unbelievably slowly and you can only rotate it if you bump into it, you can't move it along the X, Y or Z axis'.