Slow boot with external SSD after APFS

Hello!

I have a iMac 27" 5k 3,4ghz 2017. I installed an external SSD Samsung 860 Evo 256gb to avoid losing the Apple 1yr warranty if I opened my iMac by myself.

So when I was using HFS+ my iMac used to start, then 2 seconds with a completely empty screen(black) looking for the boot SSD, then the Apple logo appear and it would take another 18 seconds for the system to start completely.

Total boot time = ~20secs.

Then, I upgraded my OS to High Sierra and the system converted my HFS+ system to APFS and.... When I turn on the computer the screen stills black for 2 minutes "looking for my external boot SSD" and then the Apple logo appear and +18secs the system starts.

Total boot time = More than 1m20s

I solved this reinstalling all the OS on a HFS+ partition and unchecking the "Convert to AFPS" option.

But now I installed the new MacOS Mojave and the installation automatically converts to AFPS without option to skip this. I made a Time Machine backup, tried to install the MacOS High Sierra again but I can't restore from the TM backup because it's the newer version backup.

My current boot time = More than 2~3minutes!!!!

So... There is any solution for fixing this "everlasting" blank screen when booting the MacOS High Sierra/Mojave with external SSD and APFS format?

I've researched about this, but I can't find anything to solve it. So, I hope someone here may help me!

Thanks!