If as I think the problem is due to apfs, there is a possibility to install High Sierra on an internal SSD without converting the file system to apfs, first you download High Sierra from the App Store, then instead of launching it by double click you enter the following command in a Terminal window :
Code:
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO
This could allow to upgrade to high sierra without this problem of slow boot. Later, it is always possible to convert the internal SSD to apfs using recovery partition (I hope !).