So how would i identify which dashboard query is slow .
Browser dev tools can show a panel with pending network requests etc if you're trying to figure out the browser state.
Kibana log files will help with the Kibana server but you're best off asking these questions in the Kibana forum.
Elasticsearch has slow query logs, hot_threads and various CAT apis to show the state of the elastic servers. Marvel/monitoring will show a lot of these stats and OS level ones too like CPU and IO.
So there's a number of places to look. Just how many elasticsearch users are hitting your cluster?