I saw it first hand. My manager was pushing everyone to get promoted to level 5, of the few who didn't, one was put on a pip then fired, the others worked like madmen after seeing that, still didn't get promoted and somehow moved into another group. They still haven't gotten promoted but they work insanely hard/ aren't the same people. I guess it depends on the group, but I was told that timelines for promotion to level 5 are an unwritten rule.