Change effective on 19 April 2017
Heroku Preboot, which ensures new web dynos are started before existing ones are terminated, has been changed to account for custom boot timeout settings. The Preboot window for an app will be dynamically set to the sum of the app’s boot timeout plus 2 minutes. Previously, this window was statically set to 3 minutes for all apps. This change will take effect within the next 24 hours.
For more information, see our Dev Center guide on Preboot.