Instrument your application with Google Client libraries and analyze the traces in Cloud Trace to pinpoint the cause of the slow response.

Deploy the Profiler package on the VMs and review the data in Cloud Profiler to determine which part of the application is slowing down.

Implement the logging agent on the VMs and examine the logs in Cloud Logging to find out what's causing the application to respond slowly.

Install the Debugger agent on the VMs and use Cloud Profiler to analyze the data and identify the slow aspect of the application.