With the release of the Unity 5 version of DFC, the performance of the game under Linux seems to have slipped. The difference is profound enough that at lowest settings I can barely make Book 4 playable, but playable it was (phew!).
For reference, here are my specs:
OS: Linux Mint 17.3
CPU: Intel Core i7 4790K
RAM: 8GB (I cannot remember the timings right now)
Disk: Mechanical hard drive
Graphics: Intel HD Graphics 4600 (i915 driver, proprietary driver built into kernel)
... did I forget something important
The funny thing is that the game plays pretty well (read: "wayyyy better") under Wine, which should be slower since it is going through the compatibility layer and is also 32-bit. I am uploading a couple videos to YouTube that show the significant difference in performance. I'll update this post with links.
Could it be that there is a different effect being used in the Linux build that is slower or something like that. I can use Wine for now going forward, but it's just odd.
So if you are running Linux and your performance has dropped, give Wine a try and see how you fare. And let us know in this thread, so we can try to figure it out.
Video 1 - Just the menu
Video 2 - Spoilers?!