Partial initialization has nothing to do with the difference when you attach a spinner. All partial initialization does is rearranges boot items. You get to desktop faster but then the same things still have to load and now you wait even longer as they are loading with the OS already up.
Your delay is being caused before it even gets to the boot sector. Look in your BIOS under boot tab and disable everything but your boot drive so its not looking on the spinner also disable CSM but this may cause you issues if you didnt load the OS with it set to CSM disabled.
The caveat to this is you will have to go set it all back to the default values if you want to boot from the USB, DVD or any other device.
One more alternative is to scrap the spinner and use an SSD instead or store your media on a NAS.
If it was me Id just leave it. Not worth the hassle of 7 seconds. You've already expended enough time chasing 7 seconds to get a years worth by simply checking the messages on your phone instead of obsessing over the splash screen being up a little longer. This is one thing I've just never understood. 5 minutes to boot, OK legitimate gripe. 37 seconds instead of 30? In the grand scheme of things a few more seconds on boot has nothing to do with performance, using a spinner instead of an SSD has a monumental hit on performance.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein