There is a CHZ20.vlx around somewhere, I do not recall everything it does or how fast it would be.
What exactly is the time on a basic drawing?
As mentioned, cannot really tell any specific issues without the full LISP.
I have noticed FLATTEN actually can take a long time on a drawing with lots of objects, even if it is completely z=0 already.
FLATTEN and FLATSHOT are mostly useless, IMHO.
I would start with just a simple z=0 LISP, then run different LISPs to specifically target other objects like Regions and Blocks.
I am not sure if the arc issue have ever been solved, you may have to look around and see.
Can you post a sample drawing?