Since the installation of Mojave my iMac (21" retina 2017) has a very slow boot from the external USB SSD Mojave OS.
I have an internal drive 5400rpm with El Capitán and BootCamp, by disconnecting the external SSD it boots fast from both El Capitán and Windows (as fast as this disk allows) but once I connect the external SSD drive all boots are slow (may be 5 minutes until working).
It takes just a lot to show the Startup manager (after pressing option while booting) and then more to show the Apple logo.
With High Sierra external SSD was ver fast, I have tried:
- With a second USB SSD with Mojave (prerelease OS): same result.
- Performed diagnostics, Disk utility, etc, all is fine.
- Formatted all drives and reinstalled anything in both drives, internal with Capitán and BootCamp and external with Mojave: same result.
I have checked USB drive speed and seems normal for an USB SSD. Awaking from sleep takes more than normal but I have not noticed any lag in applications once it is working.
I'm not 100% sure Mojave start it but I already noticed something abnormal with the testing Mojave SSD.