How to reproduce:

1. Copy a Unity install to the Downloads folder
2. Start Unity
3. Open a new project
4. After some time loading, Unity freezes with these errors being repeated in the log console:

UnityShaderCompiler(32348,0x7fff91637340) malloc: *** mach_vm_map(size=9223372036854775808) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug

This is due to some processes being put in quarantine by Mac OS.

Reproducible on 2018.2 and early 2018.3 builds on Mac OS High Sierra
Fixed in 2018.3.0a1 (2018.2 will be patched soon)