Could be memory speed, yeah. Turning on and off resizable BAR had no effect for me.
From what I understand DX12 games will handle shader caching on their own, as opposed to DX11 where it was more predictable what was happening. Maybe by default devs cache to system RAM or something, which depending on your system setup ends up being worse than DX11 caches to disk. Consoles don't care because they have unified memory. Don't really get it but that's my theory. Maybe BAR support is just fundamentally broken or no one's using it properly.
Whatever the case is 1.01 was apparently better, so they should just roll back their changes.
Unified memory doesn't affect shader compilation. These are different issues.
On a console, because it's standardized hardware and software, devs can compile shaders and ship them with the game. So the game just loads them, as any other asset.
On PC, using low level APIs, because of different GPUs and drivers. It's necessary to compile shaders for each system, or there will be graphical issues.
Some game engines have good tools to pre-compile shaders. Like Unreal Engine. But not all devs know how to use it properly. So we get games that run perfectly, without stutter, like Gears 5. And other that are a stutter fest.
The other issue, is asset streaming. PC and consoles have different memory configurations. Each have their advantages and disadvantages.
And require deferent approaches to optimize. In this case, it's the lack of technical capability to optimize for PC, that is cause of the problem. From Software really lacks the technical know how, to optimize their games.
This can be seen on consoles, where there are issues with their games. But is very obvious on PC, where their games are very lacking.