NFS performance with 10GBase-T

Hi everyone,

I am coming here from
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=2771971160

TL;DR:

* I have a FreeBSD14 Server and Client with an Intel X540 (ix) adaptor each.
* I am trying to improve the NFS throughput.
* I get 1160 MiB/s via nc, but only ~200 MiB/s via NFS.
* Increasing rsize and wsize to 1 MiB increases throughput to 395 MiB/s
* But a Linux client achieves 560-600 MiB/s with any rsize.
* The mtu is set to 9000 but this doesn't make a difference for the 
FreeBSD client (it does make a difference for Linux).

I assume < 400 MiB/s is not the expected performance? Do you have any 
advice on debugging this?

Thank you for your help,
Hannes