Hi,
in most cases, you can't compare desktop games with VR (except you maybe run a 4k display). VR is much(!) more demanding with high resolution and high framerates.
In addition to this, VaM (in its current version) is very bad in using your hardware to its full potential. Even I can't have scenes with more than one-two people, multiple light sources, high physics hair and high poly clothes, ... , running without reprojection on a Valve Index with a brand new Ryzen 5800x and a RX 6900xt.
The bad thing is, that it won't matter that much, if you spend 2000$ more on your hardware... VaM will most probably not run that much faster to justify the money. As long as we finally have the future new VaM-Version, beware of high demanding downloaded scenes. Many of them are only playable at desktop mode IMHO or with a very strong stomache. First thing to try: delete all light sources except of one.
Saying this, 15/20 fps seems awfull low. Is this only with some high demanding scenes, or even at the start with one figure and one light? To have a comparisation: I could run VaM with low demanding scenes (only one light source, one person, decent hair and clothes, low settings and 100% SS) at a decent frame rate, so that I just don't got motion sick on my former PC (idle 90-120fps, in motion 45-60fps). I had a I7 4790K and a gtx980ti (~RTX 2060) before.
There is a very good thread somewhere around here, which scenes and settings are making VaM run slow. This is something that every VaM-fan should read and know.