I have large superfast fibre, which has a good solid line rate (76Mb down, 20Mb up). However my download throughput is much lower (50Mb-55Mb). I don't see any alarming errors on the router stats. I have had the line drop as low as 3.5Mb, but a router restart sorted that out. I swapped out the HG633 for another HG633, and the throughput seems settled at 50Mb. Upstream throughput is around 18Mb which I consider normal, and working correctly.
I called customer support, and was fobbed off with "there is a 30% margin between the sync rate and throughput, so your line is normal". As a result of this, I couldn't get an engineer out to check the line.
The router is plugged directly into the test socket, as I don't use any of the phone extensions in the house. The computer is connected via ethernet cable, so is not WiFi related.
What can I do to escalate this?