I Windows 10 installed on an external SSD that is connected to a USB 3.2 Gen 1 port, which I guess used to be called USB 3.0 and has an advertised speed of 5 Gb/s. The SSD has a speed of 20 Gb/s.
I haven't measured it, but the boot time is definitely upwards to a minute or so. I am also having slow speeds when backing up and restoring images which I do on a regular basis for the application of this whole setup. Restoring a 40GB backup takes about 10 minutes, instead of 1 minute if the transfer rate was at 5 Gb/s, according to a calculator I used.
I dont have any problems within the OS, so it sounds like the USB 3.0 drivers aren't being loaded during the image and boot process which could explain the slow times.