SELECT 
  gp.*, 
  p.company_id 
FROM 
  cscart_product_variation_group_products AS gp 
  LEFT JOIN cscart_products AS p ON gp.product_id = p.product_id 
WHERE 
  gp.group_id IN (15939)

Query time 0.00114

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": ["idx_group_id"],
          "key": "idx_group_id",
          "key_length": "3",
          "used_key_parts": ["group_id"],
          "ref": ["const"],
          "rows": 39,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["dev_db.gp.product_id"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}