SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'en' 
WHERE 
  pfv.variant_id IN (
    81139, 81141, 81142, 81144, 81150, 81151, 
    81160, 81161, 81164, 81165, 81166, 
    81168, 81170, 81171, 81172, 81173, 
    81174, 81175, 81176, 81177, 81178, 
    81179, 81180, 81181, 81182, 82009, 
    82010, 82011, 82012, 82013, 82014, 
    82015, 82016, 82020, 82021, 82022, 
    82023, 82024, 82025, 82026, 82027, 
    82028, 82029, 82030, 82031, 82032, 
    82033, 82034, 82035
  )

Query time 0.00173

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 49,
          "filtered": 100,
          "index_condition": "pfv.variant_id in (81139,81141,81142,81144,81150,81151,81160,81161,81164,81165,81166,81168,81170,81171,81172,81173,81174,81175,81176,81177,81178,81179,81180,81181,81182,82009,82010,82011,82012,82013,82014,82015,82016,82020,82021,82022,82023,82024,82025,82026,82027,82028,82029,82030,82031,82032,82033,82034,82035)"
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "ref": ["dev_db.pfv.variant_id", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "pfvd.lang_code = 'en'"
        }
      }
    ]
  }
}