@Spyder99 Would be EE tech but below some info for you!
1. FTTP is a contended service, up to 32 users can be sharing the 2.5Gb backhaul, so you can never expect the full speed all the time, you may get 900mb when there are fewer people using it. I expect that you are sharing the backhaul with many users. If everyone was using it fully, you may only get 78mbs. Its called statistical multiplexing, which relies on the fact that all users are not utilising their connection fully, all of the time.
BT quote up to 900mb, so you are likely to get much less than that during peak times.
Speed tests pass very little data, so normally give a much higher speed.
2. The max OR connect to a splitter is 30 ( 32 is the splitter maximum but policy is 30 ) not every CBT port provided is likely to have a customer using it , so unless on a ‘new site’ that has no alternatives to OR FTTP the actual number on a splitter is likely to be way less , OR currently have about a 30% take up, so maybe 10 users per splitter , plus the majority don’t take 900Mb but slower profiles , and the chances of those ‘on line ‘ at any one time all and doing something intensive, rather than browsing / Netflix that may be consuming less than 30-100Mb , is slim , that’s why there is a 700Mb minimum speed guarantee on 900Mb …..the 2.4Gb will be plenty ,you would have to be incredibly unlucky to have any consistent congestion.
If you suspect PON congestion, try at a time when there won’t be much activity, late evening or early morning .
Although you have tried somethings to ‘ isolate’ the problem , the most obvious thing to do ( that you haven’t apparently tried ) is use the BT router , without doing that , you haven’t really proved anything , your third party router may great , but even great routers can be mis configured or faulty
