Cross rack network latency in AWS

I have VMs running on bare-metal instances. Each bare-metal instance is in a separate rack by design (for fault tolerance). The bandwidth is 25GbE however, the response time between the hosts is so high that I need multiple streams to consume that bandwidth.

Compared to my local on-prem lab I need many more streams to get the observed throughput close to the theoretical bandwidth of 25GbE

# iperf StreamsAWS ThroughputOn-Prem Throughput
14.8 Gbit21.4 Gbit
29 Gbit22 Gbit
418 Gbit22.5
823 Gbit23 Gbit
Difference in throughput for a 25GbE network on-premises Vs AWS cloud (inter-rack)