Imo Cerny's claims actually make sense when it comes to the way they are doing boost after reading DF's article. Fundamentally, consoles can't be in a situation where they have variable performance in games from console to console due to thermal throttling, power issues and etc. Every console needs to be able to run the game the exact same way as every other console since that is one of the main purposes of a console over PC. Sony mentions this in the DF article:
"According to Sony, all PS5 consoles process the same workloads with the same performance level in any environment, no matter what the ambient temperature may be." - DF
I think the Variable/Boost behaviour comes from the states that ps5 will be running. I am pretty sure Cerny and the rest has done their research and determined that the ps5 will have various states that it can run on that will be repeatable from console to console depending on what the developers want.
As an example. If the CPU is running at 3.5ghz at all 8 cores, the GPU can run at blah clock but below 2.23ghz. If the GPU is running at 2.23ghz then the CPU will run at blah clock but below 3.5ghz at all 8 cores. If both need to run at max performance at the same time, they will run slower than 3.5 and 2.23 but we don't know by how much. Mind you that Cerny says it's not much of a performance dip regardless. So if a game is very GPU bound but not very CPU bound, the ps5's GPU can boost to 2.23ghz all day and vice versa but the ps5 can't have both. The benefit and the main difference of Xbox is because it's not "boosting," it's able to run at it's advertised performance at all times for it's GPU and it's CPU is determined by whether or not the devs want SMT/HyperThreading.
So CPU and GPU load for the XSX doesn't affect each other but it does for the Ps5.
"When that worst case game arrives, it will run at a lower clock speed. But not too much lower, to reduce power by 10 per cent it only takes a couple of percent reduction in frequency, so I'd expect any downclocking to be pretty minor," - Cerny
Least that's what I got out of it anyway.
This is how I interpreted it hearing Cerny talk. There is some talk here and on GAF/Reeee that the PS5 is actually locked in terms of performance and can run in “boost” mode at all times... that’s obviously false, or else why have variable speeds that can affect performance? A CPU heavy game will affect the GPU performance. A GPU heavy game will affect the CPU. Those that utilize both will affect both. Meaning the more demanding a game is, the less performance you get out of the PS5 because it has to run at lower speeds.
It was interesting when he said you’d see at most around a 10% drop in performance, that would get you right around 9-9.2TF which was what the original leaks had PS5 at all along. So maybe they did see XSX specs and overclock it? Will be interesting to see the price and cooling.
Either way both consoles should be capable of pushing out good stuff. XSX will just look better.