I found this out by accident on a return trip from Reading to Heathrow this weekend. It adds about 8-10 minutes to the journey. I have to say I'm finding it quite infuriating and also very surprising as the bus is already only marginally competitive with the train - from Reading to Terminal 5 the bus was usually quicker, but to Terminal 2 it can be quicker to take a train to Hayes & Harlington followed by a local bus. This change is making the bus lose its competitiveness for Terminal 5, compared to taking a train to West Drayton followed by a local bus - and at over 2.5 times the price it's getting very hard to justify taking the bus! The convenience of having no changes is now really the only thing going for it.