Wasted my whole day today, trying to diagnose exactly the same issue. The further I go into the campaign, the longer it takes for the game to start. It was almost unnoticeable at first, but now I'm only in Grymforge, and starting the game takes about 6 minutes. Of which 5 minutes 10 seconds (on average) the screen isn't updated, the spinner stops, and the game is doing seemingly nothing.
For me, switching from DirectX to Vulkan solved the problem. Moreover, when I first started it on Vulkan, it had a similar delay (although noticeably shorter), and an additional message "Rebuilding the shader cache. This may take a while" appeared on the loading screen.
On the next start, the shader rebuild message was on screen for maybe 2-3 seconds, and the game started as fast as it's supposed to.
Looks like in DirectX mode the game rebuilds this shader cache from scratch every time it starts, and Vulkan has an optimisation check for this. Considering that rebuild runs on a single CPU core (judging by the PerfMon trace), no wonder it takes so much time.