If the time it takes for Win10 to fire up bothers you, consider not turning it off, pretty much ever. Lots of devices come that way out of the box -- it's not that big of a deal. Put Win10 into that low power state where, except for a really minimal trickle of electricity, it's off for most intents and purposes.
That said, if the device has a lower end CPU, once Win10 starts you're in no way ready to get to work [or play] -- I generally put Task Mgr. up & walk away until things calm down. An SSD rather than a conventional hard disk can help, *IF* you aren't held back by the CPU. In Win10 if you go to Settings -> Apps -> Startup, or the Startup tab in Task Mgr. you can turn some stuff off that is set to start with Windows, and you'll see a rating, generally high or low impact to guide you. You can also disable or set manual start for services that automatically start with Windows using the Services app in Control Panel -> Admin. Tools. SysInternals' Autoruns is the most comprehensive tool, but WARNING, it has no failsafes and you can break things easily.
