Windows 8 is very different to Windows 7 and Vista. Some programs are not supported and after upgrading to Windows 8 you will find that some programs have even been removed! Windows 8 actually removes Media Center from the computer! The trouble is that Media Center and other programs it removes or disables are still set to start in the registry. This means that there are many errors due to missing files, which will slow down Windows startup. Fix those errors and Windows should start faster.
Go and get Autoruns for Windows and unzip it. It doesn't need installing and you just double click autoruns.exe. Here is what it looks like on a computer that has been upgraded from Windows 7 to Windows 8:
See all those startup entries highlighted in yellow? They used to start Media Center when Windows 7 was running, but Windows 8 deleted it and so the files cannot be found. There is no point in having startup entries that cannot be found, so right click each one and select Delete.
Scroll down the list and delete every one of those yellow items that say File not found. The next time you start Windows 8 it should be quicker and smoother.