(Disclosure: I'm a Quarto dev)

This is an area we want to improve in the future. Most of the JS libraries can be disabled in the project configuration. We find that our typical user prefers to have access to the features those libraries provide, but I agree with you that we should be doing better. This is a place where some guided documentation would help. We're working on it.

With that said, we dogfood Quarto pretty seriously, and consume the content from mobile devices. I admit that we use devices that are likely in the 90% percentile of speed, but website performance is something we do take into account.