Slow network between EC2 instances

I copy files from an i3.metal instance to a t3.medium instance with 16Gb sc1 EBS disk. Both located in us-west-2 (Oregon) region (though in different AZs). Sometimes I have up to 200Mb/s speed (which might be a limitation of an SSD on i3.metal, so this is a different question), but sometimes only 30Mb/s! See the graph from CloudWatch: https://i.stack.imgur.com/JiiS8.png

i3.metal is said to have 3125 Mb/s network, t3.medium is said to have up to 625 Mb/s network. sc1 disk baseline IO throughput should be 192Mb/s. But clearly, something is limiting my speed. Could anyone suggest what this can be?

Edited by: mmatrosov on May 21, 2021 7:47 AM