NVMe read latency is in the 10-100µs range for 128kB blocks. S3 is about 100ms. That's 3-4 OOMs. The threshold where the total read duration starts to dominate latency would be somewhere in the dozens to hundreds of megabytes, not kilobytes.

I agree, it's an oddly low threshold. The latency differential of NFS vs. S3 is a couple OOMs, so a threshold of ~10MB seems more appropriate to me. Perhaps it's set intentionally low to avoid racking up immense EFS bills? Setting it higher would effectively mean getting billed $0.03/GB for a huge fraction of reads, which is untenable for most people's applications.

< NVMe read latency is in the 10-100µs range for 128kB blocks. S3 is about 100ms. That's 3-4 OOMs.

Aren't you comparing local in-process latency to network latency? That's multiple OOM right there.