This page demonstrates a vulnerability in multiple browsers, wherein the Windows Performance Counter Frequency can be extracted. From this number, it is oftentimes possible to detect virtualization, and if not - to determine the CPU clock speed. The full details can be found in the advisory.

The prerequisites are:
- Windows operating system, version 7 or above
One of the following browsers (or browser families):
- Microsoft Internet Explorer 10-11 (in general, Trident-based browsers with Trident version 6 or above)
- Microsoft Edge (in general, EdgeHTML-based browsers)
- Mozilla Firefox 34.0-40.x (in general Gecko-based browsers with Gecko version 34.0-40.x). The issue is fixed in Firefox 41 (Gecko 41).

Data about your browser and O/S is obtained via WhichBrowser library: