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

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"

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