Z370 Killer i5 8400 @ 800GHz

800mhz for low load is normal and means your CPU's power saving is working. The frequency (speed) will go up when you need it. This is not the same as your base frequency. It does not effect performance in any way.

If power saving is not something you care about you can set your minimum speed via the Windows power plan. It uses a percentage rating, typically it is set to 5% by default.

Windows Power Settings -> Change Plan Settings -> Change Advanced Power Settings -> Processor Power Management -> Minimum Processor State -> Set to desired lowest % value

This will set your lowest power saving state to a higher figure, which means your system will use more power (higher electric bill). It won't hurt the system in any way so feel free to play with it and figure out what setting suits your needs best. I typically bump it up from 5% to about 30% on older/slower systems which can help things feel a little snappier going from idle to load but new systems (like your i5 8400) make the change much faster, I doubt you will notice any difference.