NIP-45 Event Count would be great to reduce relay/client bandwidth usage when counting reactions and replies (instead of, for each thread, client having to request and download many events just to count them client-side). Specially considering the inter-relay sync capabilities of strfry that could enable more accurate counts.

I've never used lmdb so don't know how hard it would be to add event count support. If there is speed problems, maybe doing a regular db event fetch up to a limit such as 200 events and counting relay-side would already be great. Even if it effectively caps counts to 200, currently most threads don't have that much engagement.