Your information is very out of date. Microsoft long ago worked with the Newtonsoft.Json creator when introducing their native .NET replacement for Newtonsoft.Json. Performance in the new set of libraries is excellent as they rely heavily on the newer performance primitives like Utf8String, Span, etc.
No, it never does. There are better containers than dynamic, that do not have its performance drawbacks and have way better UX. The fact that this does not raise an eyebrow is a legitimate concern over the state of the codebase that uses it!
