It’s very tricky to talk about ‘VXGI performance hit’ and put it in numbers.
If you want it to rival Lightmass (and fail while trying) you can have it cripple even the most powerful of rigs.
If you want something like lpv-quality levels, it’s capable of matching that too.
Personally that’s what I really like about VXGI. You can pretty much customize it to almost all your needs.