GoCoax local internet speed problem

Hello,

I have a problem with local LAN speed. If I run iperf with this command

iperf3.exe -c 192.168.1.245

I get speeds around 125 Mbit/s

When I run iperf with this command

iperf3.exe -c 192.168.1.245 -P 8

I get speeds of around 773 Mbit/s

Which is already much better and would be enough for me...

PHY Rates I actually have like this: +-3600 Mbps (both of them) And the center line values +-700

Now to the problem, I measure this speed on this route: NAS-Router-GoCoax (in)-GoCoax (out)-PC

But if I start dragging files to the NAS with windows explorer (to Synology) I get only 15 MB/s = 120 Mbit/s

Do you know where the problem might be?

Thank you!