nostr query is as fast as an in memory btree lookup. Out of process relay will never compare, especially with the serialization overhead

Only if the data happens to be already mapped to memory, right? If it needs to go to the disk, performance drops. Even mmap is slow compared to any serialization.

Sync: blocking, async non-blocking. I'm interested in this for a specific API design. Communicating with other Android apps is necessarily async. But I'll think about it. I see the advantage of using a device relay but I'm unsure of the UX. Are you going to ask your users to download and continually run citrine in order to use your app? It will also depend on the overlap - for Amethyst and Zapstore it's probably none