FreeBSD 13: Bootloader very slow on some Mac models

**Describe the bug**
Booting FreeBSD 13 based helloSystem 0.7.0 from external USB media is _very_ slow at the bootloader stage (**minutes** instead of seconds) during which the screen may be blank. People may think the system is not booting at all. It appears like the bootloader is running only at a tiny fraction of the normal speed.

**Affected models**
* iMac 7,1
* MacBook 4,1
* MacBookPro10,1
* Macmini5,1

**Not affected**
* Macmini4,1 (firmware `MM41.88Z.0047.B00.1802021546 02/02/18`)

TODO: Test with FreeBSD 12 based helloSystem 0.6.0