BigPipe

Sends pages in a way that allows browsers to show them much faster. First sends the cacheable parts of the page, then the dynamic/uncacheable parts. Uses the BigPipe technique.

See screencast at https://www.youtube.com/watch?v=JwzX0Qv6u3A.

Will be proposed for Drupal 8.1.x core inclusion at #2469431: BigPipe for auth users: first send+render the cheap parts of the page, then the expensive parts, depending on how well this module works in the real world during the 8.0.x cycle.
As of Feb 29, 2016, is included in Drupal 8.1.x, shipping in Drupal 8.1.0 and later! See #2469431: BigPipe for auth users: first send+render the cheap parts of the page, then the expensive parts.

See http://wimleers.com/blog/drupal-8.1-bigpipe-module-experimental for details — in short: this contrib module is being maintained only for sites still on Drupal 8.0.

How does this work?

1. During rendering, the personalized parts are turned into placeholders.
2. By default, Drupal 8 uses the Single Flush strategy (aka "traditional") for replacing the placeholders. i.e. we don't send a response until we've replaced all placeholders.
3. The BigPipe module introduces a new strategy, that allows us to flush the initial page first, and then stream the replacements for the placeholders.
4. This results in hugely improved front-end/perceived performance (watch the 40-second screencast above).

Sounds complex?

Here's a simple explanation with the concrete benefits: https://www.youtube.com/embed/X34MFjXNzzk.