What would this tool figure out from the JSON file? TBH, I've never noticed we had anything like this in our rather simple setup.

It checks if any of the deprecated features are used. 4.0 removes classic queue mirroring, changes the underlying metadata storage to khepri and introduces many improvements. Instead of asking many questions, it analyses the definition file locally in the browser.

Do they have Erlang OTP JIT enabled on RabbitMQ? Why does it still feel so slow?

Yes, JIT is enabled. Slowness is quite relative to the use case. Which part do you find slow?

Yes, we are!!!