Hello Vaishnavi,
You are to some extent right, when you express concern about the performance of an app engine application using CloudSQL 2nd Generation: network latency for connections between App Engine standard environment and Second Generation instances is approximately double the latency for connections to First Generation instances. Also, at present, there is no support for query caching.
Besides Cloud SQL, you have the choice of other storage products, that may better suit your purpose.
Regarding scaling, it may be worthwhile bringing to mind that Datastore is designed with scalability in mind, whereas MySQL does not scale well in practice.
This being said, you may design your application from bottom up with scalability in mind, following useful generally applicable good design practices: designing for scale.