Back in the late 90's I worked as a starter at a public golf course outside Chicago for a numer of summers. The first few years the spacing between tee times was 7 minutes. The course was constantly jammed up and rounds routinely were close to 5 hrs. Brutal. Then, management decided to move to 9 minute intervals. There was a huge difference. Pace of play quickened and guys were playing sub 4 hour rounds in the early morning and 4 hour 15 minute rounds by mid-morning/early afternoon.
With 9 minute intervals, I literally had to stand on the tee in front of groups to prevent them from teeing off until it was their time. As soon as the group ahead had cleared the fairway and were on their way to the green, the next group wanted to tee off. I usually had to hold them 2 or 3 more minutes, with an empty fairway. Guys would always ask why they couldn't tee off because "we can't reach them."
It's not about staying ahead of the group behind you, it's about keeping up with the group in front of you. If everyone did that, pace of play would be much better.
I don't think pace of play will ever be reasonable on the PGA Tour, no matter how many minutes are between groups. In many cases, guys are playing for their cards and trying to make a check and almost no one ever gets penalized for slow play. If slow play was penalized with strokes, on a regular basis, for everyone, pace would speed up. But, I can't see the PGA Tour really ever doing that.