nothing to do with voltages. If your voltages were wrong, the processor either wouldnt work or would blow up.
The thing to look at are bottlenecks in your system. Maybe you have small ram, or maybe your hard drive came from an older pc and is only ata66. Maybe you have the older syle ribbon on your hard drive?
There are lots of things to check. Look at what your virtual ram is set to, maybe it's too low.
What sort of applications slow the pc down?