if you go to the network tab of your browsers developer tools to grab the msearch request payload:
Then you can use that request payload in dev tools:
Inspect the payload to see if it looks accurate, then see how long the request takes coming back from elasticsearch. Hopefully this will help you know whether the issue is with Kibana's request, or that elasticsearch is too slow. If it's an issue with elasticsearch, you can ask for help in that channel as it may be a configuration error with your nodes and shards.