Why would there be a delay at Exeter, if you ran SWR services to Plymouth?
Because of the need to reverse?
But Exeter would be a major intermediate stop on any Waterloo - Plymouth service, so you'd have a dwell of a couple of minutes regardless of which direction the train was carrying on in
Looking at Bradford Interchange at this time tomorrow (i.e. standard Monday to Friday timetable), trains only stop for two or three minutes including the reversal, which seems reasonable for serving a major city station in the middle of the journey
Train information at Bradford Interchange around 1201 on 20/09/2021. From Realtime Trains, an independent source of train running info for Great Britain.
www.realtimetrains.co.uk
(link shows the Monday lunchtime arrivals/departures at Bradford Interchange, commonly two minutes in duration)
Whereas taking a service that needs to fit precisely into the carefully timed congested corridor from Basingstoke into Waterloo and subjecting it to the vagaries of single track line through Dartmoor....
that's more likely to cause delays in my book - would only take one thing to go wrong and the whole thing would fall apart (if you absolutely had to slot into a narrow window through Woking etc)