How fast can a LN payment be over TOR? I wonder how fast a LN payment can be over TOR if all possible optimizations have been implemented?
Basically as fast as a regular payment. The encrypted onion is being transported from node to node and will most likely leave the TOR network pretty quickly as many connections in lightning happen with direct IP connections.

The path calculation is happening locally on your node and does not need communication with your peers at the time when the payment is sent (before to download gossip information you need to talk to your peers) 

And then of course the first hop sending the onion to your partner will now happen via TOR and go trough a couple of TOR nodes taking potentially a more complex path than a direct IP package to your peer would take. 

I would argue that this extra IP traffic from TOR can be neglected in comparison to forwarding the onion and probing several paths.