alt Hacker Newsless than 60ns loopback latency
There are some Ethernet switches with 4ns latency, and those do more than just sending and receiving, so there's clearly still an order of magnitude of improvement still available. 4ns is basically ~40 cycles of the bit clock for 10G Ethernet.
The other funny bit is that one-way PCIe latency is 250ns-ish (don't quote me on the exact numbers), which imposes a hard 1us constraint on latency between two hosts.