Now the avg is around 42kmph. At most it can be brought to 50kmph by reducing the running time to 17hrs 20min, but beyond that it might not be possible, as most of the path is in a single line section (except GTL-HPT).
Even though the running time is 15hrs, it might not attract the passengers because in one direction. In the return direction, this train departs on Fridays ex.VSG.
If the running time is 15hrs, then the train would been into SF category.