Find the centre of interaction
For an interactive application, place the compute near the users who need the most responsive experience. For a background worker, the important route may be to a database or external API instead. Different workloads can justify different regions.
Measure a representative journey
Test complete application requests, not only ICMP ping. Include DNS resolution, TLS setup, database access and typical payload sizes where relevant. Sample busy and quiet periods and record more than a single average.
Consider placement obligations
Identify where personal or regulated data may be stored and accessed. Review the contracting entity and subprocessors as well as the physical location. Seek qualified advice when a specific legal requirement affects your project.
Keep recovery independent
If a second region is part of recovery, confirm that it is operationally separate and that the application can switch to it. Test DNS, credentials, backup access and data restoration before an outage forces the first attempt.