Elastic Search performance degraded after veriosn update from ES 1.4.2 to ES 7.9

[GauriA (Gauri)]

We want to upgrade the OLD ES 1.4.2 setup to new ES 7.9. For that we are testing the search performance. I am testing the search performance by querying the search queries returning different no. of documents simultaneously by 5 different clients(REST API).I am using scroll API to get the results by calling it one after other to get all the matching results. I am just iterating through the results before making next scroll request. Some queries returning very large data, some returns medium data. There is a query which returns 90% of documents of the index. This scroll API query takes 2 hrs to complete in ES 1.4.2 and same query takes about 11 hrs to complete in ES 7.9. I was expecting performance increase after upgrading to ES 7.9 but found huge performance degradation. Anyone knows the reason behind this?. All the default settings I m using for both the ES versions. Please provide help here. Thanks

Sample Query : #Query 1 : http://localhost:9200/cgh_hs_hg19_20170821/_search?scroll=1m

{
    "size": 1000,
   
     
    "query" : {
      "match_phrase" : {
         "num_hits" : "1"
      }
   }
}