Hey,
it is very unlikely that msearch is faster than search - slower might make sense as an msearch with more than one request would need for all requests to finish.
In this case it might be that your query is hitting different shards and thus also different nodes. If those nodes have different loads this might result in this.
You could set the preference parameter, to test this out https://www.elastic.co/guide/en/elasticsearch/reference/6.1/search-request-preference.html
--Alex