Your first example with a separate variable is actually slower. In both cases you have the $post variable already in your scope, and reading from that doesn’t cost anything. But if you create a copy and assign that to a new variable, you are using more resources. Just reading an existing variable doesn’t cost any time.
In real life, it doesn’t matter. The difference is so small that you probably can’t even see it in performance tests unless you do that with thousands of different variables at the same request.
What you should go for is readability. Using $post->ID makes it a bit easier to see where that value is coming from than with $post_id. In an IDE you can click on the $post part of $post->ID and get some information about this object, like the other variables and maybe available methods.