Instrument your application with Google Client libraries and view the traces in Cloud Trace to pinpoint the aspect of the application causing the slow response.

Install the logging agent on the VMs and sift through the logs in Cloud Logging to find out what's slowing down the application.

Install the Debugger agent on the VMs and examine the data in Cloud Profiler to identify the bottleneck in the application.

Install the Profiler package on the VMs and analyze the data in Cloud Profiler to uncover the cause of the slow response.