Look at the thresher stage by stage. Crop first meets the spinning cylinder and concave, where it is rubbed and struck, this is the threshing stage, and its outcome depends on how fast the cylinder's rim is moving, that is the peripheral speed.
After threshing, the loosened grain and broken straw fall onto the concave grate and travel over the straw walkers, this stage is separation, driven mostly by walker shake and grate design, not cylinder rim speed.
Then the fan blows air through falling material on the sieves, this stage is cleaning, controlled by fan speed and airflow, again a separate system from the cylinder.
Since the peripheral speed acts right at the threshing stage, the parameter it changes is threshing efficiency.
\[ \text{Affected parameter} = \text{Threshing efficiency} \]