performance drop on high bandwidth capacity links

test framework:
hyper-v  freebsd vm 14.3-RELEASE-p5
hyper-v linux vm

test 1:
- ipfw kmod unloaded
- iperf3 receiver on freebsd vm
- iperf3 client: any other vm connected to the same hyper-v switch or the
windows 11 host itself

[  5]   0.00-10.00  sec  22.1 GBytes  19.0 Gbits/sec    0            sender
[  5]   0.00-10.00  sec  22.1 GBytes  19.0 Gbits/sec
 receiver

test 2:
same conditions but with ipfw loaded, no other rules but default to accept:

[  5]   0.00-10.00  sec  7.58 GBytes  6.51 Gbits/sec    0            sender
[  5]   0.00-10.00  sec  7.57 GBytes  6.50 Gbits/sec
 receiver



i ran the same (ipv4) iperf3 test  against a linux vm under the same
conditions, with and without nftables. i got no (significant) performance
drop, even with a stateful firewall.

everything is default-configured, no special configurations were applied to
any (sub)systems.
there is no packet loss, no retries.

is this normal?

thank you,
adrian