Solr offers several different join implementations which can be switched off providing the "method" local-param on JoinQuery's. Each of these implementations has different performance characteristics and can behave very differently depending on a user's data and use case.
When joins are used internally as a part of JSON Faceting's "join" domain-transform though, users have no way to specify which implementation they would like to use. We should correct this by adding a "method" property to the join domain-transform. This will let user's choose the join that's most performant for their use case during JSON Facet requests.