Yeah the boot is slow, but maybe its because of the custom boot for apfs.
http://dosdude1.com/highsierra/
- Please note that if you use APFS, you will not have a bootable Recovery partition.
-- It is recommended that you only use APFS if the target drive is an SSD.
-- If you decide to use APFS, a custom booting method will be installed by the post-install tool, as the firmware of these unsupported machines does not natively support booting from APFS volumes. It is not quite as clean as native booting, but will not cause any issues while running High Sierra.
If you don't mind, you may try turn off TRIM for testing purpose and see if the boot time will be back to normal.
For some reason, using APFS and TRIM together in HS seems cause slow boot (and few more issues).