Effective tension is calculated from friction resistances of the idlers, the weight of belt and material, and any lift, using standard conveyor design equations. Speed simply is not one of the inputs to that calculation.
What speed does change is the power needed to run the conveyor, because power is tension multiplied by velocity.
\[P = T_e \times V\]
Since \(T_e\) stays the same regardless of \(V\), only the power output scales with speed, not the tension itself.
\[\boxed{\text{Effective tension doesn't depend on belt speed}}\]