SELECT 
  cscart_product_features.feature_id, 
  cscart_product_features.feature_type, 
  cscart_product_features_descriptions.description as name, 
  CASE WHEN (
    cscart_product_features.feature_type IN ('M', 'S', 'E')
  ) THEN GROUP_CONCAT(
    cscart_product_feature_variant_descriptions.variant
  ) ELSE CASE WHEN (
    cscart_product_features.feature_type IN ('O')
  ) THEN cscart_product_features_values.value_int ELSE cscart_product_features_values.value END END as val 
FROM 
  cscart_product_features 
  INNER JOIN cscart_product_features_descriptions ON cscart_product_features_descriptions.feature_id = cscart_product_features.feature_id 
  AND cscart_product_features_descriptions.lang_code = 'en' 
  INNER JOIN cscart_product_features_values ON cscart_product_features_values.feature_id = cscart_product_features_descriptions.feature_id 
  AND cscart_product_features_values.product_id = 13502 
  AND cscart_product_features_values.lang_code = 'en' 
  LEFT JOIN cscart_product_feature_variants ON cscart_product_feature_variants.variant_id = cscart_product_features_values.variant_id 
  LEFT JOIN cscart_product_feature_variant_descriptions ON cscart_product_feature_variants.variant_id = cscart_product_feature_variant_descriptions.variant_id 
  AND cscart_product_feature_variant_descriptions.lang_code = 'en' 
GROUP BY 
  cscart_product_features.feature_id

Query time 0.00082

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "25.36"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "6.50"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_product_features_values",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY",
              "lang_code",
              "product_id",
              "fpl",
              "idx_product_feature_variant_id",
              "fl"
            ],
            "key": "product_id",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "const"
            ],
            "rows_examined_per_scan": 13,
            "rows_produced_per_join": 6,
            "filtered": "50.00",
            "index_condition": "(`cscart_db`.`cscart_product_features_values`.`lang_code` = 'en')",
            "cost_info": {
              "read_cost": "8.47",
              "eval_cost": "0.65",
              "prefix_cost": "9.77",
              "data_read_per_join": "260"
            },
            "used_columns": [
              "feature_id",
              "product_id",
              "variant_id",
              "value",
              "value_int",
              "lang_code"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_features",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "feature_id"
            ],
            "key_length": "3",
            "ref": [
              "cscart_db.cscart_product_features_values.feature_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 6,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "1.62",
              "eval_cost": "0.65",
              "prefix_cost": "12.04",
              "data_read_per_join": "2K"
            },
            "used_columns": [
              "feature_id",
              "feature_type"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_features_descriptions",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "feature_id",
              "lang_code"
            ],
            "key_length": "9",
            "ref": [
              "cscart_db.cscart_product_features_values.feature_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 6,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "1.62",
              "eval_cost": "0.65",
              "prefix_cost": "14.32",
              "data_read_per_join": "14K"
            },
            "used_columns": [
              "feature_id",
              "description",
              "lang_code"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_feature_variants",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "variant_feature_idx",
              "idx_var_feat_pos"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "variant_id"
            ],
            "key_length": "3",
            "ref": [
              "cscart_db.cscart_product_features_values.variant_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 6,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "1.62",
              "eval_cost": "0.65",
              "prefix_cost": "16.59",
              "data_read_per_join": "7K"
            },
            "used_columns": [
              "variant_id"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_feature_variant_descriptions",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "variant_id",
              "lang_code"
            ],
            "key_length": "9",
            "ref": [
              "cscart_db.cscart_product_feature_variants.variant_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 6,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "1.62",
              "eval_cost": "0.65",
              "prefix_cost": "18.87",
              "data_read_per_join": "48K"
            },
            "used_columns": [
              "variant_id",
              "variant",
              "lang_code"
            ]
          }
        }
      ]
    }
  }
}