See this article:
http://www.archicadwiki.com/Multiprocessing
particularly the section titled "Is ArchiCAD multi-threaded?"
While some parts of AC benefit from multiple cores, as noted there, most of AC runs with a single core, so what you're seeing is normal.
If your 2D is slow, see this article:
http://www.archicadwiki.com/2dSpeed
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB