After a spike in traffic to your application on AppEngine, users report ongoing high latency. What is the most suitable first step to mitigate this?

Modifying the App Engine configuration to have additional idle instances is the most suitable first step to mitigate high latency issues after a spike in traffic. App Engine operates with a scaling mechanism that allows it to automatically adjust the number of instances based on incoming traffic. By increasing the number of idle instances, the application can better handle sudden increases in user requests without causing delays.

This approach allows App Engine to pre-warm additional instances, ensuring that they are ready to serve traffic as it comes in, which helps reduce latency experienced by users during high-demand periods. This solution is particularly effective for applications that experience sudden, unpredictable spikes in traffic, as it can provide immediate relief without requiring significant architectural changes or migrations.

Other options, while potentially beneficial in certain contexts, don't address the immediate need to handle increased traffic. Increasing the number of idle instances directly responds to the growth in user requests, making it a practical first step in performance management for App Engine applications.