Improve App Engine performance by reducing entity size

Question

The objective is to reduce the CPU cost and response time for a piece of code that runs very often and must db.get() several hundred keys each time.

Does this even work

Answer 1

if I want to reduce the size of my entities, is it necessary to migrate the old entities to ones with the new definition?

Yes. The GAE data store is just a big key-value store, that doesn't know anything about your model definitions. So the old values will be the old values until you put new values in!