Index sorting with two order values in the same field

[joaoantao (João Filipe)]

I am trying to improve queries in one index with ~ 125 million documents and 3 shards. Most of the queries hitting this index have a sort order for a given field with values ascending and descending.

Currently the index settings are the following:

{
  "joao-2023-03-08_2" : {
    "settings" : {
      "index" : {
        "refresh_interval" : "60s",
        "number_of_shards" : "3",
        "provided_name" : "joao-2023-03-08_2",
        "creation_date" : "1678304042926",
        "sort" : {
          "field" : "a_field" ,
          "order" : "asc"
        },
        "number_of_replicas" : "1",
        "uuid" : "0-8O_yOYQ_iAxC2UUpaJGB",
        "version" : {
          "created" : "135248027"
        }
      }
    }
  }
}

The queries with sort a_field=desc are slow. I know it is possible to modify the configuration and pass the same field (a_field) in the array with the desc value. Doing this does not seem correct, but I am not sure how internally the segments will be created.

Is there anything I could read or do to improve this scenario?

Thanks in advance!