I would not be surprised if FreeBSD NFS is slower than Linux NFS, but 10x slower is too weird to be correct. Have you used the same NFS version, e.g. NFSv4, on both FreeBSD and Linux?

I have used for many years file servers on FreeBSD, servicing a great number of users and they certainly were not slower than Linux and they had perfect reliability. It is true however, that I have used Samba, not NFS.

I have also used NFS in a few cases, but I have not run benchmarks. I mean that I have not tested intensive random accesses, but I have just copied entire disks through NFS and that worked at the speed limit imposed by a 1 Gb/s Ethernet link, so at least for sequential transfers NFS did not seem to have any speed problems.

The speed of NFS also depends on the speed of the file system used on the server. If you have tested a FreeBSD with ZFS versus a Linux with XFS or EXT4, than your benchmark might not reflect anything about FreeBSD vs. Linux, but only about ZFS. ZFS is significantly slower than XFS or EXT4, regardless if it is used by FreeBSD or by Linux.

Nobody uses ZFS for speed, but only when the extra features provided by ZFS are desired. ZFS is still faster than BTRFS, but not by so much as XFS/EXT4 are faster than ZFS.

On FreeBSD, its older file system, UFS, is faster than ZFS, though not as fast as XFS/EXT4. But if you use NVMe SSDs on the file server, the speed of NFS should be mostly limited by Ethernet, not by the file system of the server.