Hello, Ryuu,
yes, 4K needs optimization.
Could you try exactly with this scene (the corridor you see in my previous message)?
I installed the new DB (21 July) to my 1 year old laptop. It's an i7 8750H (six cores). It's 2.5X faster than one of my old desktop PC (i7 970- six cores too).
In fact, I have almost zero seconds "Precomp time" using UHD Cache and 6-7 seconds with 4K Cache, when I had 15 seconds with my i7 970 using 4K and only 1 second with UHD (exactly 2.5X slower).
In IR, with laptop, I have 1 FPS with 4K and 10 (yes, 10!) FPS with UHD Cache.
So, I think something is wrong in the 4K Cache.
Obviously, with a 64 CPU core, 4K cache will be super fast, but that' not the point ^__^