I'm not a huge fan of the Elvis operator, because I think the responsibility for making all template-accessed properties be in a sane state is the controller's responsibility, and the template shouldn't have to be defensive about it.
That being said, I can't conceive of it having any measurable performance impact. It just gets inlined as
isValueBlank(user) ? null : user.name, and wouldn't get reevaluated until
user changes, which would happen when the AF observable emits.
Can you describe in a bit more detail what changes you made to the app code and its dependencies between when you liked the performance and now? Were you using Angular 2 before and now are using Angular 4? Are you using the lazy page loading code splitting system now and weren't before? Are all your pages and components in a single module? Any other significant version bumps in potentially relevant dependencies?