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 (656, 643, 648, 766) 
  AND pfv.product_id IN (
    2685, 2623, 2455, 8398, 2304, 2374, 2438, 
    2499, 2581, 2110, 8484, 8485, 8488, 
    8486
  ) 
  AND pfv.lang_code = 'en'

Query time 0.00326

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "33.34"
    },
    "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": "idx_product_feature_variant_id",
          "used_key_parts": [
            "product_id",
            "feature_id",
            "lang_code"
          ],
          "key_length": "12",
          "rows_examined_per_scan": 57,
          "rows_produced_per_join": 57,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "6.52",
            "eval_cost": "5.70",
            "prefix_cost": "12.22",
            "data_read_per_join": "44K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`pfv`.`feature_id` in (656,643,648,766)) and (`softwarepirmam_hewadelivard_cscart_4`.`pfv`.`product_id` in (2685,2623,2455,8398,2304,2374,2438,2499,2581,2110,8484,8485,8488,8486)) and (`softwarepirmam_hewadelivard_cscart_4`.`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": [
            "softwarepirmam_hewadelivard_cscart_4.pfv.variant_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 3,
          "filtered": "5.88",
          "cost_info": {
            "read_cost": "14.25",
            "eval_cost": "0.33",
            "prefix_cost": "32.17",
            "data_read_per_join": "3K"
          },
          "used_columns": [
            "variant_id",
            "feature_id",
            "position"
          ],
          "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`fv`.`feature_id` = `softwarepirmam_hewadelivard_cscart_4`.`pfv`.`feature_id`)"
        }
      },
      {
        "table": {
          "table_name": "fvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "softwarepirmam_hewadelivard_cscart_4.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 3,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.84",
            "eval_cost": "0.33",
            "prefix_cost": "33.34",
            "data_read_per_join": "10K"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}