The problem is being exposed by titan level graphics cards putting so powerful a load especially at 1080p on the processor and memory and PCIe controller that it overloads the data fabric.
Different games all put different levels of loads on the computer. Explosions require many more calculations than plain blue sky for example. 150fps explosions that a Titan/1080 level card can produce many more calculations for the physical properties of the explosion together with the processing required to draw 150 frames in a second puts more load on computer rendering the same game at 80fps that you would get with a 1060 for example.
Data Fabric bandwidth is directly proportional to memory frequency. Data Fabric is basically a network between elements on the chip. Those functions include CCX thread switching, memory access, PCIe controller access and all require timely access over the network or performance is impacted.
Switching threads between CCX modules relies on the Data fabric being available on demand to switch the thread to the other CCX module.
When you overload a network you get contention for the available resources. Just like if you go to the bank to see a teller. If there are three tellers in the bank and you are first, you go straight in, do your business with teller number one and leave. Everything is quick and no contention. If you are customer number 4 in the queue, the first three get served quickly and you have to wait for the next available teller, slowing you down. The more people in the queue, the longer you have to wait.
If the DF is overloaded and the thread wants to switch when the DF is busy, it has to wait.
16 threads running at once with SMT all require more bandwidth to access the amount of memory that is required than if you only use 8 threads all accessing memory at once.
The Data Fabric clock runds at 1/2 the frequency of the memory frequency. The more clock cycles available to perform instructions and tasks will make what every you are using faster and will increase the point where contention becomes an issue. Faster memory provides the faster frequency to the Data Fabric. Basically the same as adding teller 4 and teller 5 to the bank counter means 5 peopla at a time dont have to wait in a queue..
Because of the reliance on the data fabric, the observed CCX thread switching, scheduling and SMT on and off performance differences are all symptoms of the underlying Data Fabric contention issues, not the cause of slow Ryzen frame rates with 1080TI at 1080p. You do not see the same issues with a 1060 or 1070.
The Data Fabric architecture is completely new and not seen before in a computer Processor. New creates different challenges to what went before but to address a challenge, you have to find it first. The majority of the Tech media/industry including it seems enginneers at AMD are trying to apply principles that they learned from using Intel or FX chips to this new architecture and have not yet worked out that the Intel principles are only 95% the same. They have gotten themselves stuck fixating on windows schedulers and thread switching. Those things are demonstrating anomalies but the anomalies are a symptom of something else, not the primary cause. The reason is not windows software it is the underlying framework that they are ignoring because it is in that last 5% that is not being considered. The tests have only been done using the high powered Nvidia cards. It could actually be being caused by something in the Nvidia drivers.
It is not possible to be the scheduler being the root of the problem because if it was, you would also see the same anomalies in Cinebench and other non gaming benchmarks. But there is no evidence of that. SMT would not be shown as being more efficient than Intel Hyperthreading in non gaming benchmarks
Faster memory is improving things. The way things are headed, together with faster memory, I have a feeling that an nvidia driver or a bios setting will be found that reduces Graphic card's contribution to the load on the Data Fabric to below the level where contention becomes an issue and the problem will be solved.