I have just tried it: FreeCAD still freezes MacOS, but only for a few seconds.
It used to freeze the system for about a minute. Also, when I open a file I will get a "beachball" cursor. It is not as bad as a system freeze, but it shouldn't happen (it's the OS feedback of an unresponsive app. After a while the OS would ask to force quit the app).
Another info that will be important for the upcoming MacOS 10.15: the binaries are not signed. For now (up until 10.14), the user can still run them by setting the security settings low, doing a right-click to start FreeCAD and then accepting a security dialog. I believe this won't be possible with 10.15.
Update: It still freezes for about a minute on startup. I don't know why it only froze for a few seconds on first startup.
OS: macOS 10.14
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.17107 (Git)
Build type: Release
Python version: 3.7.3
Qt version: 5.9.7
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/Germany (en_DE)