DL360 G6 - Why does W2K12R2 report its processor speed varying?

[Vague Assumption]

When I consult W2K12R2's task manager, and look at the performance tab, I see a difference between the processor's maximum speed (2.53 GHz) and its current speed (about 1.60).  I assume that if I wanted the processor to run at full blast, I would need to alter the BIOS, right?

Which setting would I use?  the processor is an Intel XEON E5540.