I could be mistaken, but I believe it downloads some of that from the satellite. It takes time to download some of that and "sync" with the satellites.
I see that also. It's loading stuff in the background. The amount of time you stated is significant. I'd get a sat reload from Sirius xm.. Call them and ask them to send the signal... If that doesn't clear it up perhaps a reload of the latest software. I think this thing has the capability to glitch itself with good code and require a reload. I've seen that three times. My load time is usually thirty seconds.
Also. Do you have a usb device connected? Could be indexing your music causing the unit to hang for a bit while it does so on boot up
I recently had the same symptoms as the OP described, a one time event. I guessed I was getting a refresh. My other car usually indicates what is happening when this occurs, I saw none on my 8.4AN.