Complaining that BIOS boot is 15 seconds? Seriously, I remember the days when booting took 2 minutes at least...
So, things that could slow it down. If Windows is installed as MBR/legacy or GPT/EFI mode, that's the first thing that comes to mind.
Also the more controllers active on the motherboard, the slower boot will be. iGPU, sound, SATA, Ethernet, and any other addons on the motherboard will each need to be checked to POST.
Also you may want to look for BIOS updates and driver updates to make sure those aren't slowing you down.
Also the more updates and apps installed in the OS that needs to be loaded on boot will slow things down.
Otherwise I guess it's just memory and storage. Check that your memory is in dual channel mode, check that your SATA is in ACHI and not legacy, and the usual.
Also, maybe it's time for a refresh of the OS if you've had it a long time?
...how many times did I say "also"? I must be tired...