My 1660 ti and Ryzen 7 2700x build has been working great for almost a year until I decided to move back to my old home and now the clock speeds have been incredibly underwhelming. Normally it hovers around 3.5-3.7 but it's been dropping below 3.0 regularly for a bit now. Nothing was out of place, and no settings have been tinkered with. Is it a power issue?
Usually when CPU performance drops that means something has gone wrong with cooling. First thing I would do is check for failed fans or big dust buildup (I have seen some incredible dust bunnies inside friends' PCs). Maybe check CPU temperature under load.
If you want to verify if you are getting good power, run OCCT from OCBASE dot COM. It is free software that will test your Power Supply. You can also run HWINFO in real time while it is testing to see your voltage.