Heroku will terminate a request connection if no data is sent back to the client within 30 seconds. From Heroku’s Router – HTTP timeouts documentation:
HTTP requests have an initial 30-second window in which the web process must return response data (either the completed response or some amount of response data to indicate that the process is active). Processes that do not send response data within the initial 30-second window will see an H12 error in their logs.