for size>=4, I get a latency of about 30 microseconds as expected, however for
size<4, communication costs increase massively, and latency now appears to be
1ms!

my guess would be some form of interrupt/message coalescing.

Secondly, can anyone suggest what's going on, or where I can go for more
information.

what are your nics?  have you looked at driver-level settings (ethtool, I 
guess)?
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