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 (550) 
  AND pfv.product_id IN (
    300, 303, 304, 305, 306, 307, 308, 309, 
    310, 311, 312, 313, 314, 315, 316
  ) 
  AND pfv.lang_code = 'en'

Query time 0.00095

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.89"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "variant_id",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 7,
          "rows_produced_per_join": 0,
          "filtered": "2.41",
          "using_index": true,
          "cost_info": {
            "read_cost": "1.28",
            "eval_cost": "0.03",
            "prefix_cost": "2.68",
            "data_read_per_join": "133"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "((`atulecarter_atul_demo1`.`pfv`.`product_id` in (300,303,304,305,306,307,308,309,310,311,312,313,314,315,316)) and (`atulecarter_atul_demo1`.`pfv`.`lang_code` = 'en'))"
        }
      },
      {
        "table": {
          "table_name": "fv",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "feature_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "ref": [
            "atulecarter_atul_demo1.pfv.variant_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "7.73",
          "cost_info": {
            "read_cost": "0.17",
            "eval_cost": "0.00",
            "prefix_cost": "2.88",
            "data_read_per_join": "15"
          },
          "used_columns": [
            "variant_id",
            "feature_id",
            "position"
          ],
          "attached_condition": "(`atulecarter_atul_demo1`.`fv`.`feature_id` = 550)"
        }
      },
      {
        "table": {
          "table_name": "fvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "atulecarter_atul_demo1.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.01",
            "eval_cost": "0.00",
            "prefix_cost": "2.90",
            "data_read_per_join": "40"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}