SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
FROM 
  cscart_storefronts AS storefronts 
  LEFT JOIN cscart_storefronts_currencies AS currencies ON storefronts.storefront_id = currencies.storefront_id 
WHERE 
  1 = 1 
  AND (
    currencies.currency_id IN ('1')
  )

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.00"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "storefronts",
          "access_type": "system",
          "possible_keys": [
            "PRIMARY"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.20",
            "prefix_cost": "0.00",
            "data_read_per_join": "2K"
          },
          "used_columns": [
            "storefront_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "currencies",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY",
            "idx_storefront_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "storefront_id",
            "currency_id"
          ],
          "key_length": "8",
          "ref": [
            "const",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.20",
            "prefix_cost": "0.00",
            "data_read_per_join": "16"
          },
          "used_columns": [
            "storefront_id",
            "currency_id"
          ]
        }
      }
    ]
  }
}