Cobra951: System particulars: Skylake i5 6500, 6GB GTX1060, 16GB 2133MHz DDR4 RAM. Game running from Toshiba 7200rpm HDD, saves on Samsung SATA III SSD.
What I found is that the stuttering I was getting was entirely due to the unlocked frame rate. The fps got so high in small areas that my GPU temp would rise to over 80 C. I discovered this by accident when I alt-tabbed to the desktop for game-unrelated reasons, and I realized I wasn't running Firestorm (the Zotac app for Nvidia cards, with my custom fan-speed curve). Running it immediately sent the fan speed way high, which made me look at the alarming GPU temp.
So, what seems to be happening is that when the CPU has no trouble feeding the GPU enough draw calls for an excessive frame rate, the GPU froths itself into a lather and gets way hot. After setting vsync, not only did the temps stay below 50 C, but the frame pacing smoothed out to a solid 60 fps. Stutters are gone.
Conclusion: Runic should consider enabling vsync by default, or at least advising users to enable it if possible. On modern game systems, the GPU otherwise can get seriously overdriven in some areas of the game.
I believe this is a separate issue to the one that Runic are investigating.
I use an i7, NVIDIA 1070. My temps sit comfortably at around 60/75 (GPU/CPU) and I still get extreme FPS drops when running around the world regardless of vsync or FPS lock. It appears other 1070 users and some 1080 users are also affected.