This doesn't answer your question, especially as my experience is not time related, but may
throw some light on it as it shows the very variable behaviour you can get from the TTB test.
I have a basic gigabit switch which I connect to a BT HomeHub3 and to my computer.
With switch connected to the gigabit port on the HH3 I saw TTB tests around 10Mbps
With switch connected to a 100Mbps port on the HH3 I saw TTB tests around 70Mbps
With computer connected directly to gigabit port on the HH3 I saw TTB tests around 70Mbps
In all cases the HTTPx6 tests were full speed around 70Mbps
All figures approximate.
BT Infinity 2, thinking of moving to PlusNet