I know the tracks around HW have speed restrictions [mps of 50kph, is it?]. But that doesn't mean you run a bullock cart on that section. This 'express' train takes 2+ hours for 27 km at an average of hold your breath, 12 kph. In comparasion, the slowest BG train is DC passenger[Dhanbad Chandrapura] which does ~13 kph due to underground mine fires around that area.
Also, 8 hours 45 minutes for MB-LKO? That time is enough to do NDLS-LKO.