It's just where the tightest structures (physically, electrically, or timing-wise) are found. The AMD cores and uncore are, I'm guessing, fairly well tuned to the process by skilled people. The bolt-on stuff often isn't so well done, particularly if an end-client project manager or beancounter is driving the schedule. So it's almost certain to happen if the added stuff has very complex paths, and you wouldn't be going to the effort of adding silicon if you didn't need it to do something interesting....
So, no proof, but it wouldn't be any kind of surprise if this were part of the problem.
So, no proof, but it wouldn't be any kind of surprise if this were part of the problem.