SELECT 
  DISTINCT pc.category_id, 
  count(DISTINCT pc.product_id) as total_products 
FROM 
  netting_products AS products 
  LEFT JOIN netting_products_categories AS pc ON products.product_id = pc.product_id 
WHERE 
  1 
  AND products.product_id IN (408) 
GROUP BY 
  pc.category_id

Query time 0.00035

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "filesort": {
      "sort_key": "pc.category_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "products",
              "access_type": "const",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "rows": 1,
              "filtered": 100,
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "pc",
              "access_type": "ref",
              "possible_keys": ["pt"],
              "key": "pt",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["const"],
              "rows": 2,
              "filtered": 100
            }
          }
        ]
      }
    }
  }
}