Oh, I don't doubt that there are people trying to run this thing on a potato just because it looks like it's from 1996. It's still weird that it runs as bad as it does on modern systems as well.
On Steam you said that all the the gravity and transformation maths might strain the CPU. Are you familiar with Outer Wilds? I admittedly didn't get very far in your demo, but I didn't notice it doing anything that Outer Wilds doesn't also do and that one runs just fine on my PC. It's really puzzling that my system is struggling so much with this.
Is there anything I can do to find out what could be the cause?
As a side note: You're probably overestimating how willing the average Steam user is to provide feedback on their own. Today there are a couple of new threads and once again they all complain about bad performance. Please don't take this as me shitting on your parade. This is me trying to be constructive and helpful because your game looks cool as hell.