There's two ways of undervolting.
- Full manual like the guy in your video is doing. This shouldn't affect even 1% lows. In fact it should improve them initially because you're achieving a more stable clock speed because you're not constantly bumping against the power limit ceiling.
- Leaving everything on auto and sliding a "voltage offset" slider. That's going to reduce your clock speed because the GPU is still riding along the "stock" frequency/ voltage curve.