The Mac mini has a very good value for money if you need raw performance in a small silent package. Frequently available for between $399 - $499 discounted.
A VPS that can perform like a Mac mini will likely cost the same as a Mac mini in 12 months time.
I picked one up to replace my prior mini that I spent 4x the amount of money on. It's an absolute speed demon.
Openclaw is running via api. The reason people are bying separate machines is for security isolation and 24/7 power — performance is irrelevant.