Some of the comments in this thread are really something.
The issue with SU9 has nothing to do with better graphics or more complexity as some in here are claiming. SU9 did nothing in regards of graphical enhancements. It looks exactly the same as SU6 after the Hotfix.
The issue is as simple as SU8: Smooth and high FPS, SU9: Stutters and lower FPS.
I never had any performance issues prior to SU9 except for WU3 which has been fixed pretty fast.
Asobo definitely has messed something up with their “performance optimisation”. FPS on the ground are as worse as it can be and off screen caching seems to be gone completely.
The only good thing about this is, that they are “investigating” so I have high hopes for SU10. However I hope that they aren’t just turning down graphics and call it a day as SU8 was perfectly fine - visually and performance wise.
Edit: Corresponding thread in the bugs section with lots of pictures and videos highlighting the issues for those who are interested.