Be it PAM, or whatever OpenBSD is doing, the session setup kills performance, whether you're re-using the SSH connection or not, every time you start something within that connection.
Now obviously for long running stuff, that doesn't matter as much as the total overhead. But if you're doing long running ssh you're probably using SSH for its remote terminal purposes and you don't care if it takes 0.5 seconds or 1 second before you can do anything. And if you want file transfer, we already had a HTTP/3 version of that - it's called HTTP/3.
Ansible, for example, performs really poorly in my experience precisely because of this overhead.
Which is why I ended up writing my own mini-ansible which instead runs a remote command executor which can be used to run commands remotely without the session cost.