I suspect Theresa May is harking back to the 1980s when there were indeed fast services from Twyford and Maidenhead, running non-stop from Maidenhead. I used to catch them.
However, they used to be class 50 hauled and would join the mainline just east of Maidenhead. Being class 50 hauled and capable of 100 mph they could keep up with most traffic on the fast line. Also, in those days there was no Heathrow Express so less traffic on the approach to London.
Not sure how you'd do it now.
Its done with 125's and 180's and the ones with 165/6 which 90 mph have a much quicker acceleration than a class 50 so in many ways a faster service, also quite a few depart from plat 2 so no messin about crossing over