Why doesn't my computer work at full speed?

Solution 1:

Are you sure it's not just saving energy? Put some load with youtube HD videos and look if it doesn't speed up. CPU-Z will report the changes in real time. My core i5 goes from 1200 MHz to 2800 MHz every time it needs more speed.

Solution 2:

The Pentium 4 630 comes with SpeedStep which downclocks when the load is less to reduce power consumption/heat buildup.

SpeedStep is a trademark for a series of dynamic frequency scaling technologies, including SpeedStep, SpeedStep II, and SpeedStep III built into some Intel microprocessors that allow the clock speed of the processor to be dynamically changed (to different P-states) by software. This allows the processor to meet the instantaneous performance needs of the operation being performed, while minimizing power draw and heat dissipation. Enhanced Intel SpeedStep is sometimes abbreviated as EIST.

This is the cause for the variations