Provided you don't have any wired network issues (bad cables, wrong NIC interface settings, network loops, duplicate names etc), other features like AdGuard filters, DNS configuration etc that consume additional resources or routing rules can affect speed.

When I troubleshoot, I always start with the basics and disable all optional features to get a baseline.

Do you have any custom settings or features enabled?

EDIT - I looked through your earlier thread and it does seem like you've got a somewhat complicated setup... Plenty of variables to check. Also, you mention that you are using your ISP's box in your setup as both modem and bridge relaying to your Opal? I am not familiar with UK internet providers, but in the US they are not known for providing the best devices that plays nice with 3rd party equipment...

EDIT 2 - If I understand correctly, your Opal is in Bridge Mode with your ISP's box as the source. I would enable WiFi on the ISP box and test speed from that connection while standing in the same location that the Opal is placed. I would want to confirm that the Opal itself can get good speed from the ISP box. Because if it has trouble there, obviously everything else downstream will suffer as well.