SELECT 
  SUM(prpd.reviews_count) 
FROM 
  cscart_product_review_prepared_data AS prpd 
  RIGHT JOIN cscart_products AS prod ON prpd.product_id = prod.product_id 
  AND company_id = 98 
WHERE 
  storefront_id = 1

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "prpd",
          "access_type": "ALL",
          "possible_keys": ["PRIMARY"],
          "rows": 51,
          "filtered": 100,
          "attached_condition": "prpd.storefront_id = 1"
        }
      },
      {
        "table": {
          "table_name": "prod",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["live_clonned.prpd.product_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "prod.company_id = 98"
        }
      }
    ]
  }
}