I'm guessing that materialized views can be sped up if the distribution of the component row keys can be mirrored across tables, so that the materialization can be treated as local and no internode communication is necessary.
Having primary keys with identical node distribution keys is a recent set of commits...
Yes, although that is probably useful for only a very limited subset of use-cases for materialized views. There are, however, more general purpose improvements to the approach that are just more involved to implement (and have their own negative performance risks, that would need to be balanced)