Revert optimizations to python based object rendering which can use lots of RAM

Hi @tyagiri, @ripinder.singh,

I attempted some optimizations first in bc470825 and then a partial revert in 59ca9cf5 after local testing. But I heard you faced issues with RAM usage.

This MR reverts these optimizations completely. Please let me know if this performs better for you.

Background info: The speedup is achieved by using multiprocessing to render each object instance, this does not respect the configuration multiprocessing setting since right now we do not pass this information to the rendering module. I tried to set it to always use multiprocessing but it seems like this can cause massive resource usage.