The Rules don't cause slow play. Lost golf balls don't cause slow play. Raking bunkers and marking putts don't cause slow play. The PGA Tour doesn't cause slow play. Slow players cause slow play, plain and simple.
Slow golfers, with rare exceptions of age or infirmity, are people who either don't know or don't care what they are doing to slow everyone else down. In the first place, they have to be shown what to do, and in the second place they have to be "incentivized" to do better. It is a complex issue, and it is beyond irritating.
On a rainy morning last Saturday, I played as part of a fivesome. I was the only walker, it was cart path only on a sopping wet course that was playing LONG, and one of our group has a prosthetic leg; we finished in under 4 hours. Yesterday, I played as part of a fourball; again, I was the only walker, but it was 90* for carts. We finished in OVER 4.5 hours and were a hole and half behind and warned by the marshal on the 11th tee.
Both rounds were part of a points game, playing by the Rules, putting out, etc. But yesterday, two of the three players in my group thought that their time was more important than mine, which I find unforgivable and unbearable. (BTW, their etiquette in general is abysmal, too, but that's another story.)
It's the players, pure and simple.