Jeff: I see a possible problem with your tests that bit me before! ipferf3 is not multithreaded by default. The more capable computers probably have an interrupt rate sufficient to handle 10gig over USB (which likely multiplies the interrupt rate needed), but it's completely possible you're pushing the interrupt rate limits on the Macbook Neo and other lower powered hardware.
Any chance you could re-run with `-P 4` where 4 is the core count?
Any chance you could re-run with `-P 4` where 4 is the core count?