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I don't think there is much gains to rewrite everything in a faster language. Unless they are a very very successful company with billions of customers, it's often cheaper to scale horizontally.

Depends on if you identify it early enough really and even then halving your app tier costs could be sizable for any company. If you can get some productivity benefits at the same time from say static typing then there's even more reason to switch.

There's still a good reason to pick fast frameworks in a slow language: you can delay the inevitable for a bit, probably enough time for you to work on a rewrite or whatever.

Well if you convert everything from ORM to raw SQL, it will then be easier to extract all of that SQL and use it in a different web framework once you've measured and confirmed that your bottleneck is servicing requests in Python.