But why not do it right the first time. This is an obvious performance pitfall for people that want to adopt this feature. It is bizarre to me after the last decade has been dedicated to performance improvments.

They do this often actually. They will likely watch how people use it and tailor the design to fix that.

As for performance, in lots of use cases it's not going to be a big deal. If you are super sensitive to performance issues then you can just wait, meanwhile everyone else gets to use the new feature. You have to start somewhere and waiting to satisfy everything usually ends up with doing nothing