We sometimes get support calls from users who have many browser windows or tabs open, complaining that "the computer is slow". When we look at the available memory we see high browser utilization. Those background windows aren't "just sitting there doing nothing".

Here's an excerpt from a blog article I wrote some time ago:

"Internet Explorer (IE) is part of the Windows Operating System, and Windows is designed to be aware of all applications you are using simultaneously. It will allocate resources appropriately. This means that if you have several tabs open in IE plus several items in Outlook, Word documents, Excel sheets, etc. all at the same time, Windows will allocate memory and processor power appropriately among these applications. Your work goes smoothly…

When you use alternate browsers, that memory and processing power is no longer rationed out in order of importance or user-focus. Chrome and Firefox are designed to take as much memory and processing power as they can to give you the best possible experience within the browser. In other words, you may enjoy better performance while you’re surfing and searching the web… but once you need to shift your focus back into another program you may find your program performance or file access is intolerably slow."