Fast user switching an important feature which allows users to login into Windows PC with different account while keeping other users logged in. This allows multiple users to use computers without interrupting other user’s work. The feature is enabled by default. So, for example, If you’ve started downloading a movie and some other user arrives to use the same system, even if he log in to his user account in PC, your download will not be interrupted.

Fast user switching function may appear useful in these ways but some of you may want to disable the feature because due to this feature you might not be getting full internet or system speed as other users have left something running inside their user account which you can’t see
Read more from source