If you are on Halo, download the BT mobile app and run a speed test. It will do a throughput test to the hub and also one to the device. If it's below the estimated, it will offer you to open a fault. You can also do this via the BT website, under the help section. It may automatically fix itself though when it tests again during the day.
In my personal experience, I raised a fault due to congestion, had an engineer visit, BT said everything is OK their end, no congestion on the SVLAN etc. So I emailed Openreach, someone high up, found their email on Google. They replied and promised to look into it. A week later a senior manager from Openreach emailed me and said they discovered bandwidth capcity issues at the cabinet DSLAM. They then told me they plan to install a new network fibre and some exchange equipment in a month.
Although Openreach are supposed to be monitoring these issues and upgrading as nessasary, it was only until I pointed it out, that they investigated and then found the issue, which does suggest it's not actually monitored that well.
Each cabinet has a certain amount of fibre links back to the exchange, could be 1GB or 6GB, depends how utilized it is. During these times you have everyone using a lot of bandwidth and it just saturates the links at peak times. It causes the DSLAM to either create a huge buffer, which would slow your speed because of the way TCP congestion control works, or in severe cases, just drop the packet. Most of the time it just creates a buffer that causes high latency and the ISP may not see anything bad going on at the exchange.
After looking at AAISP status page, I can see a hell of a lot of BT upgrades from 1G to 10G going on everywhere. I'm guessing covid is having a huge effect.
Best thing to do is try raise a fault via the website, when you are getting the throughput issues. Also do a BT wholesale speed test and further diags, it will log it for them. Keep the test pictures and submit them here for the mods. You want a morning and 8-10pm one. Make sure the 8-10PM one if the final one you do though as it only logs the most recent test.
In my case, once the fault was raised, it started my right to exit date. Most users on this forum will tell you BT don't guarentee throughput, but if you do their app test, which does a throughput test to the hub (not a sync speed test) and it comes up below the guarentee, it will want you to open a fault, this is for contracts taken out after March 2019. This is all in-line with Ofcom rules (the advertised average speed ranges had to take into account congestion times). So if you are not getting the minimum, something is wrong. BT are doing the best in my opinion with this and with Halo it does start the right to exit.
It's also worth trying to get a mod to help you out. They can check the SVLAN reports for congestion and give you a quick answer if it's something they can fix or not.
