R10 - Boot timeout errors occur when a web process took longer than 60 seconds to bind to its assigned $PORT.
This error is often caused by a process being unable to reach an external resource, such as a database, or the application doing too much work, such as parsing and evaluating numerous, large code dependencies, during startup.
The default boot timeout is 60 seconds. An exception is for apps using the Java buildpack, Gradle buildpack, heroku-deploy toolbelt plugin, or Heroku Maven plugin, which will be allowed 90 seconds to bind to their assigned port.
Some related help articles:
As a last resort, if your application requires more time to boot, you may use the boot timeout tool to increase the limit. However, in general, slow boot times will make it harder to deploy your application and will make recovery from dyno failures slower, so this should be considered a temporary solution.