Your customer is receiving reports that their recently updated Google App Engine application is taking...

Work with your ISP to diagnose the problem.

Open a support ticket to ask for network capture and flow data to diagnose the problem, then roll back your application.

Roll back to an earlier known good release initially, then use Stackdriver Trace and logging to diagnose the problem in a development/test/staging environment.

Roll back to an earlier known good release, then push the release again at a quieter period to investigate. Then use Stackdriver Trace and logging to diagnose the problem.