It’s not. When you are a high frequency trader and you’ve mastered RDMA, everything around you looks slow. You are thinking in terms of 20 nanoseconds intervals, while everyone around still thinks that serving a query under a millisecond is fast.
I agree that a lot of "modern" storage stack is way too slow though, tried to find a replication-first object storage for crazy-fast random read in small number of objects last year and found none.
Why this number - this is because it’s roughly the time it takes to read 64 bytes from L3 cache. And NICs tend to be able to push data into L3 (or equivalents).
Current state of the art - look up nanoPU, from Stanford. Wire-to-wire under 100ns is not impossible, but this would normally assume pre-cooked packet, selected from a number of packets (which is not an unusual scenario in HFT).