I don't think there is a real command to do this. I know you can list all the files you're about to commit with this
git show commit_id --name-only
However, that doesn't give you an overview of what is taking long.
You can probably already see what is taking long, because of large files. Git itself is blazing fast, so if it's slow it's either big files or a slow connection. You can always run git push -v to get the output. Another command that might be useful is this one git push -v --dry-run. It won't perform anything but will give you the expected output of what actually will happen.