Painfully slow sftp between EC2 instances?

I'm using secureFTP to transfer files between two EC2 instances in the same region. Each is a t2.large instance, and the EC2 dashboard says each is healthy.

I'm seeing transfer rates of ~250KB/s, so that it will take 3 hours to transfer a 2.77GB file.

I'm using desktop client ("SecureFX", from Van Dyke). Surely this moves files directly from one instance to the other? What is the bottleneck?

Edited by: SomervilleTom on Jul 23, 2021 9:30 AM