It's exceedingly fast. The UEFI implementations on modern computers then have to scan the hard drive GPT to find the bootloaders, if there are any. UEFI Then hands off to the bootloader that then begin initializing the OS. At this point, you're probably anywhere from 5-7 seconds in in my experience. As your OS begins loading, it has to spin up the filesystem, and locate all the necessary files required to do it's thing. Most critical hardware (mouse, keyboard, video drivers) are loaded and initialized at this point.
Generally, your OS would be in some equivalent of a single-user mode at this point.
After the necessary local filesystem shenanigans are complete, non-critical device drivers (network, sound card, exotic storage media) tend to get initialized. Network drives if properly configured might be mapped, but generally (I.e. Windows) loads Most remote volumes after login, which is generally after the system enters multi-user mode.
Windows in particular is incredibly annoying because vast tracts of configuration data is tracked in the registry; an in-memory hierarchical key/value pair binary database. Most "Windows updates" tend to involve shell scripts/installers running and performing filesystem and registry updates while you're sitting there really wishing you knew why stuff was taking so long. Stuff is taking so long because the registry is a bloody mess, and your filesystem is getting polluted by Windows not cleaning up after itself.
God help you if you killed an update in the middle; as the scripts probably were not crafted with a User's convenience in mind, and therefore are likely to fail in exceedingly interesting ways.
Though I habitually try to be the most horribly behaved user in that regard, and I've only encountered a handful of updates that when interrupted had caused Bad Things(tm) to happen.
If you aren't into breaking things for fun and profit though, I'd not advise you doing the same.
Long story short; 5-7 seconds is the absolute longest I'll tolerate for boot from hardware to OS selection. After that, faster boots depend on OS developers to get things going faster.
