(I work for AWS. Opinions expressed here are my own and not necessarily those of my employer.)
> If I have a filer with 500TB of disk I should be getting 5-10GiBy/s reads
That kind of performance requires a 50-100 GbE SAN fabric. I'm unaware of any cloud provider that offers this yet, let alone an on-prem fabric that isn't extremely expensive. (Our customers can get close with the u-24tb1.metal instance type, which offers 28 Gbps dedicated EBS throughput.)
> Likewise EBS and emphemeral SSDs top out at 250MiBy/s, which is just abysmal.
There are plenty of EC2 instance types that offer 1750MiB/s of EBS throughput - see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSOptim... for a list. You'll need to stripe some 1.3TB EBS volumes together to achieve it, but it's absolutely attainable. (I did just that when I worked at a well-known YC company to build their high-throughput Kafka clusters.)
As for instance storage, I just ran a fio test on an i3.large's NVMe SSD and it performed at 80k read IOPs, 311MiB/s sustained sequential read. These too can be striped for higher performance -- I was able to achieve nearly 24GiB/sec (at 644k IOPs) on an i3.16xlarge instance.