The current version of Horizon doesn't do optimistic updates, but it's on the roadmap. Check out https://github.com/rethinkdb/horizon/issues/23. This is relatively easy because RethinkDB itself has support for notifying the client via a feed when a particular update has landed (i.e. the RethinkDB client can correlate the write with a feed message). It just didn't make it into v1, but will happen soon.