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 (
    21141, 21291, 21429, 22573, 20633, 24099, 
    19650, 22245, 22419, 22551, 20836, 
    21017, 22413, 20977, 22553, 21034, 
    21147, 25128, 21484, 21267, 23757, 
    21067
  ) 
  AND pfv.product_id IN (
    21464, 2015, 1836, 2255, 3451, 40526, 
    6555, 25180, 286, 26604, 3004, 2509, 
    294, 2576, 26608, 2492, 2302, 50233, 
    1732, 36834, 2448
  ) 
  AND pfv.lang_code = 'en'

Query time 0.03458

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": 496,
          "filtered": 0.072521202,
          "attached_condition": "pfv.feature_id in (21141,21291,21429,22573,20633,24099,19650,22245,22419,22551,20836,21017,22413,20977,22553,21034,21147,25128,21484,21267,23757,21067) and pfv.product_id in (21464,2015,1836,2255,3451,40526,6555,25180,286,26604,3004,2509,294,2576,26608,2492,2302,50233,1732,36834,2448) 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'"
        }
      }
    ]
  }
}