There were no cases of lengthy dwell times, not following any other trains so the pair of 153s struggling would be the reason
The previous 2 services were cancelled (both also pairs of 153s!) so its almost guaranteed that the majority of the time lost would be at stations. Ive noticed from my own trains that some timings down there arent 100% accurate on RTT. A pair.of 153s with no extended dwell times would generally lose around 10mins on a Manchester to Cardiff.If it’s 2x 153 (inadequate at the best of times) it could also be extended dwell times due to the amount of people struggling to get on and off, many of whom will have luggage - looks like it’s taking 2-3 minutes extra per station stop
Cardiff-Manchester services used to be run by class 155 trains (effectively 2x class 153) in the late 1980s/early 1990s, until replaced by class 158 trains. They seemed to cope with the traffic then and I don't recall them losing time. They were faster and more frequent than the previous Mk1 locomotive hauled trains (typically class 33).