Generally, a long POST time for a motherboard means it's trying to identify a component and having a hard time doing so. To find the solution, two methods.
1) You remove the connections for each component one-by-one and try booting - more than once - the first boot is to allow POST (Power On Self Test) to find any new or missing devices. Then you let Windows boot, then do a full Shut down, start again. This 2nd boot should have a fast POST if the offending device was removed. Just keep repeating until you get a fast POST.
2) Disconnect everything not essential to the system. All hard drive connections (both power and data cables), all but a single stick of RAM, any external devices including printers and all USB devices. If it POSTs slow this way, try disconnecting the mouse and keyboard as a test. (yes, Windows won't boot with no drive, that's ok since you're testing.) When adding devices, put the hard drives on last, so boot failure comes up quickly so you can power down.
The most common device for a slow boot seems to be a USB storage devices, thumb drives in particular and sometimes external HDDs.
REMEMBER that your front panel switch does not remove power from your system fully. Always pull the AC plug or use PSU on/off switch if equiped to really turn power off before disconnecting any devices on motherboard.