Well, as a Marston Vale regular I can confirm it absolutely is the case. 150s and 153s ran on time, 230s always lost time on exactly the same timings. 150/1s are probably the best because of the manual cab door which can be opened in half a second rather than the 2-3 seconds it takes for a 230 door to open (so stop to passenger doors open would be maybe 5-6 seconds, compared to 1-2 seconds on a manual cab door 150/1, then add the same on departure). A bit of it was mitigated by working from the cabs which have smaller, quicker doors, but they still lost time.
There were even bits of minor poor design that slowed it by a second or two further. For instance, the guard controls by the passenger doors face inward, rather than sideways on the 150s, so the guard would have to open the local door, alight, check it was safe then reboard before they could see where the open buttons were, wheras on a 150 they're visible from standing on the platform so that check would be maybe 1-2 seconds quicker. On a route with so many stops, seconds count - my usual example is that when hustle alarms were fitted to Merseyrail's 50x in about 1995 they had to add two minutes to Ormskirk to Liverpool times to deal with persistent late running it caused!
The difference with the others will be that the IoW units have replaced extremely sluggish and slow-doored 1930s stock (plus there are only a few stations), and the TfW stock is battery and diesel so may well be quicker off the mark than the diesel-only Marston Vale units.