Extremely high CPU usage in Windows running on Intel Pentium processor

I have a simple app (about 300 lines of code, few deps). I compile it into a Windows executable. When I run this exec. on any newer processor (I tried AMD Ryzen 5 and Intel Core i7) it works fine and the CPU usage is never higher than 0.1%, but when I run it on an older Intel Pentium processor it shoots up to about 40% which is crazy. RAM usage doesn't change. Any ideas?