things are changing recently in aws, but few years ago R-family instances had one of the fastest uplink to EBS. for example only the larges M4 instance (m4.16xlarge) has 10gbps uplink, versus R5b where it starts from 10gbps (for the lowest tier) and ends on 60gbps @ 260k IOPS. you can very easily choke EBS with DB.

EDIT: only newer C instances have comparable uplink to EBS, C5 or C4 (and some C6) starts from ~4.7gbps. just compare the EBS bandwidth column in https://aws.amazon.com/ec2/instance-types/