I've just taken a first proper look at the timetable. Can't help noticing that half the trains skip Acton Main Line and, further West, Iver and Taplow. But in general, the services that skip each of those stations tend to take just as long over the corresponding bit of track as the trains that stop there. For example, on Mon-Fri daytime, all trains take 5 minutes between West Drayton and Langley, irrespective of whether they stop at Iver. Similarly, it's usually 10 minutes from Paddington to Ealing Broadway irrespective of whether the train stops at Action Main Line (In this case with the exception that the xx:25 and xx:55 departures from Paddington skip Action Main Line but still take longer - 13 minutes).
If skipping a station doesn't save any time, then it seems a bit daft to skip it: You're just reducing the service at that station, with no benefit to anyone else.