Same has happened with Glasgow to Edinburgh, where electrification, which was meant to bring "far speedier services", means that it now takes longer from Glasgow to Edinburgh, stopping every 10 minutes or so, than the half-hourly "push-pull" express service with 2 x Class 27 did more than 50 years ago, started May 1971, with semaphore signalling in part!
The rot set in for the HST services from Paddington at privatisation, where the London-Didcot etc commuter operation was given to Thames Trains, including the substantial loco-hauled fast peak services. Immediately the long distance HST operator set about raiding this business for themselves, by stopping all the longer distance expresses at Reading, and eventually pretty much all at Didcot as well, plus even some HST calls at Slough that didn't used to be there. Essentially it was found that operating an outer-suburban style service gained more from the upper Thames Valley than it lost from longer distance passengers. There are of course some upsides. From time to time I do a day return trip from London to Swindon, which nowadays means no longer having to aim for specific trains- it is a turn up and go service. And if you are going to treat it like that, then overall, including station waiting time, the occasional express missed due to road delays getting there, etc, it can work out a bit faster.