I was upgraded to FTTC earlier this week and decided to test my speeds using the TBB speed tester. This showed that a 1x connection was running at about 20mbps with the 6x test showing about 37mbps on a 40mbps connection which I found rather strange. I was trying all sorts of things and then decided to test using Windows 8.1 rather than Linux which is my usual OS, so I connected my wife's laptop to the lan via cable (my linux is a desktop wired in anyway) and then discovered that the x1 and x6 speeds were very similar as one would expect. It seems that under Linux using any Flash based speed tester I can find the speeds for a single thread is reported as much slower than it is using Windows. I have had someone I know check this out on their Linux system (and a different ISP) with similar under reporting of x1 speeds. The problem does not seem to happen when using testers which are NOT flash based.
I am wondering how many others have a similar issue with speed tests under reporting actual speeds when running Linux, for me the problem exists using both Firefox and Chrome with both under reporting the x1 speed using the same tester. I wonder how many others have been initially misled when running such tests with Linux.