Network speed

The question is about the TCP transfer speed between my 2021 MacBook Pro 16" M1 and my linux file server. Using iperf3 I measure 941 Mbit/sec moving data from the server to the Mac (ipref3 -c <ipaddr> -R) which is a fine result but only 236Mbit/sec from the MAC to the server (ipref3 -c <ipaddr>). This is ony 1/4 the speed and as this is the most common usage I would really like to find out why and fix it if possible. I assume it is a network setting on one of the machines. Can anyone shed some light on this??

I have tried various settings of net.inet.tcp.delayed_ack and net.inet.tcp.win_scale_factor on the MAC with no apparent result impact. I then tried the test from an older PC laptop and also experienced a large reduction in speed going from the laptop to the server v.s. the server to the laptop. This seems to suggest that the issue is with the linux tcp settings.