Again sah, all depends on your hardware. When I'm talking about boot time the bios time is included because it is a part of the startup process. So from the time you press the power button on your desktop to the time it takes to get to the lock/welcome screen is what I consider as bootup time and if you have to enter password or choose user after, is what I consider as loading up Windows time, since that is dependent on how much things you have running on startup.
No doubt Windows XP is the best OS ever made period, but it's also what was built on to get us Windows 10.
I don't agree that the OS's have been getting slower though, it's actually the other way around, again provided you have the hardware.