Performance issues when ordering by subquery row count

[ibcomputing]

After some more investigation, the part that is killing the performance is:

(100 / nullif((SELECT COUNT(*) FROM Orders WHERE Orders.CustomerId=Customers.CustomerId AND (OrderStatusId=2 OR OrderStatusId=3)),0))*(SELECT COUNT(*) FROM Orders WHERE Orders.CustomerId=Customers.CustomerId AND OrderStatusId=2) AS OrderCompletionRate