I don't recall seeing any recent numbers with identical benchmarks on Mac vs PC for ArchiCAD. Not sure why there would be any significant speed difference.
Personally, I switched to Mac 4 years ago and haven't looked back once.
AC on Mac supports some things not available on AC on Windows. Just a few that I use daily:
The Help menu on Lion and above lets you type the first letters of a command and will list all menu commands that match and even highlight the menu (through all submenus) to help find infrequently used commands or to learn them. I find this huge, even as a long time user.
Since 15 (or 14??), on Mac you can search attributes in any dialog by typing characters in what you're looking for. For example, if you're setting the material for something to a Pine wood and cannot remember what the name is Wd-Pine, nn-Pine 90, whatever... just typing PIN while the material pop-up is visible highlights all materials that match and darkens all others. Wonderful. (Ditto fills, etc.)
Expose. I have a multibutton mouse button set to display all app windows - making it super fast to switch windows within AC.
The only Windows feature that I've missed, relative to AC, is related to the open/save dialogs. In windows, shortcuts are always honored ... so if you create a shortcut to a distant library folder, you can traverse it without trouble everywhere. On Mac, traversal of shortcuts seems to be more an app issue, and Graphisoft hasn't implemented it consistently everywhere. In Windows. every open/save dialog is basically a mini-Windows Explorer: you can rename/delete/move files right there. Not possible on Mac (Lion and below anyway - don't know if this has been added yet with Mountain Lion).
There are some add-ons that only work with Windows, such as the full Google Earth Connectivity add-on. (But, even that only works with 32 bit AC ... so Windows people have to install both 64 bit and 32 bit and launch 32 bit when they need that functionality. I just run it on my Mac under a virtual Windows machine [Parallels / Fusion / virtualbox].
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AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB