Offline Previews should not attempt to read offline pages database while blocking navigation. Currently, when navigating on android under poor network conditions, we may attempt to serve an Offline Preview by blocking the ongoing navigation to see if there is a copy of the page offline already stored for the user. This has a hit rate of about 2.5% and when it fails, it causes a delay before navigating to the page. This was negligible when only serving Slow2G users, but when serving 2G users, there is a slight regression in non-previews metrics.

An approach would be to store a set of hashed URLs in memory/pref and only attempt offline previews when we highly suspect the page is already stored when it matches a hash.

This solution could also be generalized to offline pages in general (i.e., for session restore, etc.) if there is a desire to serve offline pages when a page might not exist for users that are currently connected.