Can a variable frequency drive be connected to a motor driving a hammer mill to save energy?
If the mill runs at nominally 1000rpm, then running at 800rpm would almost certainly compromise its rate of milling. However, it would also consume less power. Whether you regard this as 'compromise the efficiency of the mill' is up to how you define 'efficiency'.

It might mill as finely at lower rpm than it did at rated speed, or it might produce a coarser product. Again your definition of efficiency is what determines whether this difference would be a compromise or not.