Async search does not currently support minimize roundtrip optimization when used with cross cluster search. This is a documented limitation.

With Kibana now using async searches by default, this could cause performance issues in some environments that use cross cluster search with slower/inconsistent network latency. One workaround is to enable the deprecated advanced setting courier:batchSearches in Kibana which will switch Kibana back to using the old way of querying Elasticsearch via msearch (but this option is slated for removal in 8.0).