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 (6) 
  AND pfv.product_id IN (829, 828) 
  AND pfv.lang_code = 'en'

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.48"
    },
    "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": 2,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.47",
            "eval_cost": "0.20",
            "prefix_cost": "0.67",
            "data_read_per_join": "1K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "((`betaatif_store`.`pfv`.`feature_id` = 6) and (`betaatif_store`.`pfv`.`product_id` in (829,828)) and (`betaatif_store`.`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": [
            "betaatif_store.pfv.variant_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "15.69",
          "cost_info": {
            "read_cost": "0.50",
            "eval_cost": "0.03",
            "prefix_cost": "1.37",
            "data_read_per_join": "366"
          },
          "used_columns": [
            "variant_id",
            "feature_id",
            "position"
          ],
          "attached_condition": "(`betaatif_store`.`fv`.`feature_id` = 6)"
        }
      },
      {
        "table": {
          "table_name": "fvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "betaatif_store.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 0,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.08",
            "eval_cost": "0.03",
            "prefix_cost": "1.48",
            "data_read_per_join": "971"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}