I would check your BIOS' configuration for the processor; it is the likely culprit...
The processor is rarely the problem. Running at 800MHz is the normal speed when the CPU power saving options are enabled (Speed Step, C1E, and other C States), when the CPU is idle, low load situation.
Did you try clearing the BIOS/UEFI? You could also try loading the BIOS/UEFI default settings, you should have an option in the UEFI to do that.
What are you using to monitor the CPU's speed? Do you see this CPU speed in the OS? What is the CPU's speed when you are in the UEFI/BIOS UI?
If your CPU cooler is not mounted correctly, the CPU may be thermal throttling itself. Are you monitoring the CPU temperature?
Some boards have options in the UEFI/BIOS to limit the CPU's speed, and also may have a physical switch on the board itself that limits the CPU's speed. Few boards have the physical switch to limit the CPU's speed, but that is worth checking.