Consistently higher ElastiCache latency outside of USW1 Region

The latency differences you're observing between USW1 and other US regions (USW2 and USE2) for ElastiCache could be due to several factors, even when using similar configurations and following best practices. Here are some potential explanations and suggestions:

1. Network infrastructure: Despite AWS's efforts to maintain consistent performance across regions, there can be variations in network infrastructure and routing that lead to slight differences in latency. USW1 might have some unique advantages in terms of network topology or connectivity.

2. Data center proximity: The physical distance between the EC2 instance and the ElastiCache node within an Availability Zone can vary. Even within the same AZ, there might be differences in the physical layout of data centers that could account for the observed latency differences.

3. Instance types and underlying hardware: While you're using the same configuration and sizing, there might be differences in the underlying hardware or generation of instances being used in different regions. This could contribute to slight performance variations.

4. Regional differences in ElastiCache implementation: There might be subtle differences in how ElastiCache is implemented or optimized in different regions, which could affect latency.

5. Traffic patterns and network load: Even if your instances are unloaded, there might be differences in overall network utilization or traffic patterns in different regions that could impact latency.

To address this issue:

1. Consider enabling the "Read from Replica" feature in your Redis client. This can help optimize read operations by routing requests to the most appropriate replica, potentially reducing latency.

2. Analyze CloudWatch metrics for both regions to see if there are any other performance indicators that might explain the latency difference.

3. Test with different instance types or sizes to see if the latency difference persists across various configurations.

4. If the latency difference is consistently impacting your application's performance, you might want to consider optimizing your application to work with the observed latency or explore using Global Datastore for Redis if your use case allows for it.

5. Reach out to AWS support for a more in-depth analysis of your specific setup and the observed latency differences. They might be able to provide more region-specific insights or recommendations.

Remember that while AWS strives for consistency across regions, some variation is normal and expected. As long as the latency you're experiencing meets your application's requirements, it may not necessarily indicate a problem.
Sources
Common troubleshooting steps and best practices with ElastiCache - Amazon ElastiCache
Valkey-, Memcached-, and Redis OSS-Compatible Cache – Amazon ElastiCache with Valkey- and Redis OSS-compatibility - AWS
AWS Regions and Availability Zones - Amazon ElastiCache
Amazon ElastiCache Well-Architected Lens Performance Efficiency Pillar - Amazon ElastiCache