It seems there's a performance bug in recent NVIDIA drivers that has been causing a loss of performance across likely all GPUs. Not only that, but it seems to end up using more VRAM than previous drivers too.
User HeavyHDx started a thread on the official NVIDIA forum, to describe quite a big drop in performance since the 375 driver series. So all driver updates since then would have been affected by this.
NVIDIA themselves have now commented to confirm the issue. Here's what the NVIDIA rep said about it:
This likely matches a similar performance drop observed on another Feral game, Total War: WARHAMMER.
We've been tracking it internally as bug 1963500. There was a change, introduced in our r378 branch, to the logic of allocation of certain textures, but it apparently exposed a bug in our memory manager.
Our next release branch, r390, will carry a workaround, and we're still working on finding and fixing the root cause.
So it seems to affect at least Deus Ex Mankind Divided, Total War: WARHAMMER, Company of Hereos 2 and most likely a number of other titles too.
The same NVIDIA rep also said they aim to have the 390 driver series out before the end of the year, let's hope that doesn't come with its own problems! Great to know they are aware of it and that the next driver series will have a workaround to improve it, getting Linux game performance back on track is pretty important.