just activating osl, without using it, makes any render 3 to 4 times slower
The 4.0 version also has the same issue, about 2-3 times slower. The noise pattern is very different, which is a worse one in my opinion.
Whole UI is flickering
If you have a G-Sync monitor and have G-Sync enabled, try to disable the G-Sync for Blender in the Nvidia Control Panel. This fixed my flicker issue. I have a desktop now so it shouldn't be the…