If you have a backend then you need to have a RouteRule that sends the request to the backend. However for the preflight OPTIONS call, you will have a noRoute with the Condition
<RouteRule name="NoRoute">
<Condition>request.verb == "OPTIONS" AND request.header.origin != null AND request.header.Access-Control-Request-Method != null</Condition>
</RouteRule>
Its not about performance. Its about functionality. For Pre-flight, you need to be within Apigee as thats what the client is interacting with. Your backend will be on another domain. Your client is interacting with the APIs exposed in Apigee. For Preflight - you need to have noRoute