Drupal 8.1 is accepting new features, and BigPipe is being proposed to be added as an experimental module to Drupal 8.1. BigPipe sends pages to clients in a way that allows browsers to show them much faster. It first sends the cacheable parts of the page, then the dynamic/uncacheable parts.

If you're interested or concerned, please give the BigPipe contributed module for Drupal 8.0.x a try, and even try to break it! The more confident we can be about its stability, the more likely it being part of Drupal 8.1 will be.