I just set up a new Sophos Firewall on my Dell XPS tower (testing). It was all working nice, I was getting about 230 Mbps bandwidth from fast.com. Then I enabled DoS from Intrusion Prevention --> DoS & spoof protection [tab] --> DoS settings. I clicked the checkbox for SYN flood, UDP flood, TCP flood, and ICMP/ICMPv6 flood. Then my bandwidth went to crap. The max download speeds I get are ~9 Mbps. Ammm.... I missing something? I even tried setting my WAN port to 1000 Mbps - Full Duplex. Advanced Threat Protection is also disabled.
I'm using firmware SFOS 18.5.1 MR-1-Build326 (updated from the initial build I downloaded (SFOS 18.0.5 MR-5-Build586).
I found this page but it's a little outdated so the instructions aren't the same as my GUI, plus the discussion has been locked: https://community.sophos.com/sophos-xg-firewall/f/discussions/95693/extremy-slow-internet-speed.
please disable TCP setting and try again.