FTS: indexing of pretty small number of docs goes very slow

{

 "name": "default_index_1",

 "type": "fulltext-index",

 "params": {

  "doc_config": {

   "docid_prefix_delim": "",

   "docid_regexp": "",

   "mode": "type_field",

   "type_field": "type"

  },

  "mapping": {

   "default_analyzer": "standard",

   "default_datetime_parser": "dateTimeOptional",

   "default_field": "_all",

   "default_mapping": {

    "dynamic": true,

    "enabled": true

   },

   "default_type": "_default",

   "docvalues_dynamic": true,

   "index_dynamic": true,

   "store_dynamic": false,

   "type_field": "_type"

  },

  "store": {

   "indexType": "scorch",

   "mossStoreOptions": {},

   "segmentVersion": 15

  }

 },

 "sourceType": "gocbcore",

 "sourceName": "default",

 "sourceUUID": "8ea95aa4403e85942487013cfa93da67",

 "sourceParams": {},

 "planParams": {

  "maxPartitionsPerPIndex": 171,

  "numReplicas": 1,

  "indexPartitions": 6

 },

 "uuid": "5d694a19b13c4a75"

}

[2021-02-14 10:26:56,981] - [rest_client:2314] INFO - http://172.23.123.101:8091/pools/default/buckets/default/stats?zoom=minute

[2021-02-14 10:26:57,661] - [fts_base:4421] INFO - Docs in bucket = 20000, docs in FTS index 'default_index_1': 15993

[2021-02-14 10:27:03,922] - [rest_client:2314] INFO - http://172.23.123.101:8091/pools/default/buckets/default/stats?zoom=minute

[2021-02-14 10:27:04,592] - [fts_base:4421] INFO - Docs in bucket = 20000, docs in FTS index 'default_index_1': 15993

[2021-02-14 10:27:10,831] - [rest_client:2314] INFO - http://172.23.123.101:8091/pools/default/buckets/default/stats?zoom=minute

[2021-02-14 10:27:11,512] - [fts_base:4421] INFO - Docs in bucket = 20000, docs in FTS index 'default_index_1': 15993

[2021-02-14 10:27:11,513] - [fts_base:4469] INFO - FTS index count not matching bucket count even after 20 tries: Docs in bucket = 20000, docs in FTS index 'default_index_1': 15993