It could also be nearly zero system calls per I/O operation. The kernel can poll the submission queue for new entries. This eliminates system call overhead at the cost of higher CPU utilization.
This is true, but for most intentionally optimized storage engines that syscall overhead is below the noise floor in practice, even on NVMe storage. A single core can easily drive gigabytes per second using the old Linux AIO interface.
It appears to primarily be an optimization for storage that was not well-optimized to begin with. It is not obvious that it would make high-performance storage engines faster.