Impact in performance is very low in general, and since Special K is repairing the awful implementation of DLSS/FG by Ubi it actually helps in this case.
G-Sync was broken in the previous version of Special K, now it's working perfectly fine. I'm saying this because V-Sync has always been a mess in Anvil Engine titles, it's better in AC Shadows but you still have micro-stutter.
Per the creator of Special K:
The game's Reflex / Streamline (DLSS Frame Generation) integration is
EXTREMELY broken!
- The rush job to port this game by developers who failed to read either the DLSS or Streamline documentation has created a steaming turd of a port

The abysmal NVIDIA Streamline integration lacks things such as required latency markers (
Simulation Start/End,
Input Sample,
Render Submit Begin/End) and calls to
NvAPI_D3D_Sleep (...).
- Even worse (yes, it gets worse) it has no option to use Reflex except when Frame Generation is enabled (not that this matters, considering it is the worst integration of Reflex I have ever seen!).
Problems Solved by Special K
- SK fixes a few of those problems, supplies minimal dummy data (the missing markers) to get DLSS Frame Generation to work correctly and we have uncapped framerate in menus (with the help of ersh) and cutscenes.