The fixed performance EC2 instances (in contrast to instances with burstable performance) all have dedicated physical CPU allocations, and 1:1 affinity from vCPU to the underlying CPU. Similarly, memory allocations are fixed, pre-allocated, and are aligned to the NUMA properties of the host system. When combined with both our Xen hypervisor and Nitro hypervisor, this provides practically all of the performance of the host system with regard to memory latency and bandwidth.
See: https://twitter.com/_msw_/status/1045032259189760000
With our latest 25 Gbps and 100 Gbps capable instances, EC2 to S3 traffic can make use of all of the bandwith provided to the instance.
Please send me more information about the interaction you had on the EBS side, msw at amazon dot com. All writes to EBS are durability recorded to nonvolatile storage before the write is acknowledged to the operating system running in the EC2 instance. fsync() does not necessarily force unit access, and explicit FUA / flush / barriers are not required for data durability (unlike some storage devices or cloud storage systems). Perhaps there was confusion about the question that was asked.