mutable events don’t mix well with event sourcing models. I agree with you, projections or materialized views are another layer. I wonder if the caching relay approach primal is using is effectively this second layer.

It's one way to do it, but sort of centralized because it assumes you're running a cache for the whole network. I can't imagine it would be easy to compose multiple with coverage over different parts of the network. A better interface that allows you to ad hoc query particular relays, or compose the results from multiple caches is what's needed.