When running a game on a CPU-bottlenecked PC, which graphics settings should be decreased to improve performance?
In other words, which settings usually have impact on CPU usage (rather than GPU)?
Game settings which impact the CPU usage depends on the game itself mainly and also some what depends on the hardware combination used.
Generally speaking, the graphics settings that are usually available in the games these days that impacts the CPU are -
There may be some other settings also and it varies from game to game which may impact the CPU performance. These are just the common ones but may not be present in some games. Like the Draw Distance in Assassin's Creed Unity cannot be adjusted by the player as far as I can recall, it was fixed.
Then there are some advanced graphics settings which are optimized for a particular GPU manufacturer. For example the vastly popular Physx by Nvidia works very well on their GPUs and CPU performance is not much effected by it. But this option is usually disabled in case of AMD GPUs but if enabled (I read in some forums) it is mostly taken care by the CPU and it degrades the overall performance badly.