To decrease the buffering time in YouTube first you have to use the Google Chrome web browser because you have to use a specific extension.
First you have to download the SmartVideo for YouTube extension.
After the installation is done you will be getting some additional options in the YouTube website, now play any video and then you will be able to see those additional options below your playing video.
Then you need to click on Global Preferences from the right side placed under the video and check the Smart Buffer Box.
There is also a FireFox Add-On which can be downloaded here
Now your YouTube videos will load faster even if you have a slow internet connection.