As far as I understand the new backgroundFetch API would increase the performance of the site for returning users.
If the SW is idle, when a user returns to the site it needs to reactivate before it fetches anything. This can take as long as 500ms. What the backgroundFetch aPI does is to fetch a resource in parallel to the boot up time avoiding the potentially 500ms idle time.
As far as I understand the new backgroundFetch API would increase the performance of the site for returning users.
If the SW is idle, when a user returns to the site it needs to reactivate before it fetches anything. This can take as long as 500ms. What the backgroundFetch aPI does is to fetch a resource in parallel to the boot up time avoiding the potentially 500ms idle time.