Not sure I understand the logic here: it seems to make it strictly more difficult to construct AST nodes by hand, as you now need to explicitly include undefined fields.

@mjmahone It's not AST it's GraphQLFields and GraphQLArgs.
They not intended to be constructed by hand, for example, you construct GraphQLFieldConfig by hand and pass it to GraphQLObjectType constructor that convert it to GraphQLField internally.
So these changes shouldn't affect any external code since all fields (except introspection fields that are constructed by the library itself) should belong to some type and thus should be created by GraphQLObjectType or GraphQLInterfaceType constructor.

Why it's important: Having a stable set of properties significantly speed up execution since it allows V8 to reuse shadow classes: #2089 (comment)

Since I plan to add extensions fields inside every GraphQL* type as part of #1527 I want to minimize the performance impact of this feature but having one stable set of properties per GraphQL* type.

Thanks!