SELECT 
  c.category_id, 
  c.id_path, 
  d.category, 
  COUNT(p.product_id) AS product_count 
FROM 
  cscart_categories AS c 
  JOIN cscart_products_categories AS p ON c.category_id = p.category_id 
  JOIN cscart_category_descriptions AS d ON c.category_id = d.category_id 
WHERE 
  c.id_path LIKE '828/%' 
GROUP BY 
  c.category_id, 
  d.category 
ORDER BY 
  product_count DESC 
LIMIT 
  5

Query time 0.00123

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "868.95"
    },
    "ordering_operation": {
      "using_filesort": true,
      "grouping_operation": {
        "using_temporary_table": true,
        "using_filesort": false,
        "nested_loop": [
          {
            "table": {
              "table_name": "c",
              "access_type": "range",
              "possible_keys": [
                "PRIMARY",
                "id_path",
                "p_category_id"
              ],
              "key": "id_path",
              "used_key_parts": [
                "id_path"
              ],
              "key_length": "767",
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "index_condition": "(`cscartdb`.`c`.`id_path` like '828/%')",
              "cost_info": {
                "read_cost": "0.61",
                "eval_cost": "0.10",
                "prefix_cost": "0.71",
                "data_read_per_join": "3K"
              },
              "used_columns": [
                "category_id",
                "id_path"
              ]
            }
          },
          {
            "table": {
              "table_name": "d",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "category_id"
              ],
              "key_length": "3",
              "ref": [
                "cscartdb.c.category_id"
              ],
              "rows_examined_per_scan": 13,
              "rows_produced_per_join": 13,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "3.25",
                "eval_cost": "1.30",
                "prefix_cost": "5.26",
                "data_read_per_join": "39K"
              },
              "used_columns": [
                "category_id",
                "category"
              ]
            }
          },
          {
            "table": {
              "table_name": "p",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "category_id"
              ],
              "key_length": "3",
              "ref": [
                "cscartdb.c.category_id"
              ],
              "rows_examined_per_scan": 623,
              "rows_produced_per_join": 8099,
              "filtered": "100.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "53.79",
                "eval_cost": "809.90",
                "prefix_cost": "868.95",
                "data_read_per_join": "126K"
              },
              "used_columns": [
                "product_id",
                "category_id"
              ]
            }
          }
        ]
      }
    }
  }
}