For contrast, I had an Amiga with a 120MB Maxtor SCSI drive, and power-on to looking at the loaded Workbench GUI was about 6-7 seconds. The slowest part was waiting for the drive to spin up, which seems like an acceptable reason for a delay. Warm reboots were a few seconds faster.

So no, that's not anything inherent to SCSI. It could've been either the SCSI driver being slow to initialize, or the adapter being glacial, or the drive itself taking forever to come online.