I would rather ask when it became so easy attacking Feds serve, it doesnt always happen but sometimes nowadays you see players like Murray and Berdych just ripping on Feds 2nd serve and sometimes 1st serve. I think I remember Nalbandian doing that on indoor courts at some point in Feds peak, but even then Fed had the footwork to respond to those attacks unlike nowadays.
I dont recall anyone able to attack Nadals serve like that since he improved his serve, neither Djokovic's which is strange. At the same time Feds 2nd serve can be very effective if he gets it going, his 2 main rivals; Djokovic and Nadal doesnt really have the ability to attack it.
I think the key change to the worse in Feds game is that he has so much more difficulty to turn defense to offense compared to before because of slower footwork, so if you put him on the defensive nowadays you will put him out of balance, Murray has understood this and stopped pushing against Federer but attacks alot. In fact Murray is more offensive against Federer than Nadal and Djokovic who still think pushing for mistakes still is the most effective way to beat Federer.
I think you answered here yourself about serve. I can easily find out when Rogie`s troubles with back increasing by looking on his average figures on 1st and 2nd serve speed. For example here in Shanghai he was far from OK, his 1st serve was ridiculously slow by any rate.