There’s new information here [5.1 roadmap].
No sign of NPP, but Iris seems promising.
Iris replication
The goal of Iris is to enable richer multiplayer experiences in Unreal Engine that “just work”, with larger, more interactive worlds, higher player counts, lower ping for players, and lower server costs.
This is the first iteration of the Iris replication system; the feature is considered Experimental.
Replication Performance Improvements
Enhancements to further optimize performance when using push model replication include:
- Actors with fully push-based replicated properties that change infrequently will be processed faster.
- Optimized processing for fully push-based replicated actor components.
- Added a new opt-in API that enables optimized processing for fully push-based replicated subobjects.