From their blog:
On iOS, you can enable Instant Sync in Settings to allow Instapaper to send you silent push notifications when you save a new article. The silent push notification allows the iOS app to download the new content from the server immediately, and the result is better syncing between your iOS devices and our servers.
Saving with the iOS extension just got a whole lot faster! We reworked the iOS extension (again) to shorten the time it takes to save an article to just a fraction of a second. The extension will still show for an additional two seconds in case you want to save to a specific folder, but you can dismiss it by tapping anywhere on the screen.
I’ve been using this for a little while now and it’s completely changed the way Instapaper works for me, for the better. It feels completely seamless. I remember being a little worried when Betaworks bought Instapaper, but they’re doing great stuff with the service and its companion apps.