A clean install of Windows 8 or a brand new PC will start quickly and be responsive, but sadly it won't stay like that for very long. After a few months it will start to slow down, taking longer to get to the Start screen and desktop and longer to shut down. The main cause is software that automatically starts with Windows. Prevent Windows 8 from slowing down by taking control of startup items and it will be as fast this time next year as it is right now.
The age-old way of checking for startup items is to click Start (and run in XP) and enter msconfig. That won't work in Windows 8 for several reasons. One is that there is not Start button or run menu or search box. Things are a little different now.
Right from the new Start screen you can hold down the Windows key and press R to open the Run box and then you can enter msconfig. It works from the desktop too. In fact, pressing Windows+R will switch to the desktop to open the Run box.
Msconfig is a bit different, but the Services tab is the same. Tick Hide all Microsoft services and see what is left. The fewer services that start with Windows, the faster it will be. Some services are essential and you might find security software here. Just look for services that are not essential and clear the ticks against them.
Msconfig still has a Startup tab, but it is empty, apart from a link to open Task manager. So to check for startup items you now have to use two separate utilities instead of one. (Is this progress?) Click the link to start Task Manager or right click the taskbar and select Task manager on the menu. There is now a Startup tab in Task Manager that lists the programs that start with Windows. As with services, some are essential, like security software, so look for anything that you might be able to live without. Select unwanted startup items and click Disable
A much more powerful startup manager is Autoruns. Unzip it and double click autoruns.exe. It is best to ignore the Everything tab and to click each of the other tabs in turn. This reduces the long list of startup items into smaller chunks that are easier to read.
Red highlighted items are probably not needed and a yellow highlight means a file can't be found. It is impossible to be specific about what's needed and what isn't. A good tip is to look for software you have unininstalled. Sometimes files remain and they can be disabled or files have been deleted, but Windows is still set to run them. Either way you can usually disable the entries.