SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  fv.position, 
  fvd.variant 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_feature_variants AS fv ON pfv.feature_id = fv.feature_id 
  AND pfv.variant_id = fv.variant_id 
  INNER JOIN cscart_product_feature_variant_descriptions AS fvd ON pfv.variant_id = fvd.variant_id 
  AND fvd.lang_code = 'en' 
WHERE 
  pfv.feature_id IN (
    21328, 20328, 21912, 23512, 19936, 20327, 
    20527, 21426, 21925, 22368, 23274, 
    24742, 22459, 20221, 20439, 22576, 
    21430, 21711, 24952, 23951, 23952, 
    20941
  ) 
  AND pfv.product_id IN (
    22143, 4269, 1059, 33727, 16399, 4270, 
    3669, 22534, 1041, 25850, 31949, 47009, 
    222, 18160, 3975, 2257, 1835, 1356, 
    48772, 38552, 20804
  ) 
  AND pfv.lang_code = 'en'

Query time 0.02742

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "variant_id",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["feature_id", "product_id"],
          "rows": 499,
          "filtered": 0.714672446,
          "attached_condition": "pfv.feature_id in (21328,20328,21912,23512,19936,20327,20527,21426,21925,22368,23274,24742,22459,20221,20439,22576,21430,21711,24952,23951,23952,20941) and pfv.product_id in (22143,4269,1059,33727,16399,4270,3669,22534,1041,25850,31949,47009,222,18160,3975,2257,1835,1356,48772,38552,20804) and pfv.lang_code = 'en'",
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "fv",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "feature_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "ref": ["dev_db.pfv.variant_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "fv.feature_id = pfv.feature_id"
        }
      },
      {
        "table": {
          "table_name": "fvd",
          "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": "fvd.lang_code = 'en'"
        }
      }
    ]
  }
}