Navigation Preload is an important Service Worker API allowing to offset the startup cost of Service Worker by issuing a parallel request for the main resource.
Typically, we've observed 300~400ms (1 RTT) wins on mobile with key partners (I'll try to get them to chime on this bug).
- Available in Chrome 59+
- Mozilla is supportive, tracking bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1290958)
- Edge is supportive 
- I believe that Apple was also supportive 
Navigation Preload is _one_ of the necessities for the Google Search team to ship a service worker on https://www.google.com/. Latency is a major concern, and Navigation Preload was the biggest win in several of the Chrome team’s experiments. For your consideration. Thanks.
The Google Search team have just documented why they need the Navigation Preload API: https://web.dev/google-search-sw/#service-worker-overhead.