It may be useful to help us understand what the VM, and the host too for
that matter, is doing to cause this level of utilization. While there
appears from your two data points to be a correlation, that does not mean
that one is causing the other; perhaps both are symptoms of something
else. Transferring data at 1400 Mbps sounds like a decent rate of
transfer, and even 400 Mbps is faster than most of my systems do at a
sustained level. If you're transferring files, perhaps the CPU eventually
needs to work on I/O for some reason, which would both increase the
utilization, decrease the throughput due to the latency impact. Seeing
output from 'top' may be useful in this case so we can see how the CPU is
being used at the various times on the VM and host.

--
Good luck.

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