Windows Experience Index was introduced by Microsoft with the release of Windows Vista to have a common measure of PC’s performance. It was there in Windows 7 and later in Windows 8 too. It seems Microsoft has removed this feature with the release of Windows 8.1 preview. Microsoft is yet to confirm the removal of this feature officially, but you can see it by yourself in Windows 8.1 preview.
The Windows Experience Index measures the capability of your computer’s hardware and software configuration and expresses this measurement as a number called a base score. A higher base score generally means that your computer will perform better and faster than a computer with a lower base score, especially when performing more advanced and resource-intensive tasks.
Each hardware component receives an individual subscore. Your computer’s base score is determined by the lowest subscore. For example, if the lowest subscore of an individual hardware component is 2.6, then the base score is 2.6. The base score is not an average of the combined subscores.
via: Mcakins online