There can of course be many reasons for a slow computer, but in my case, it was all the ads running in the background. I'm using Chrome, and there is an extension you can use in Chrome called AdBlock Plus. It automatically blocks the ads with their cookies and cache when you visit websites. I installed it, and it makes a huge difference. I highly recommend it, if you haven't tried it already. It's free with no strings attached.
Sometimes you may encounter a website that does not allow you to go on, unless you disable AdBlock on THEIR particular site. If you want to read the info on that site, you can disable it ONLY for that site--there is an option for that. But it's very rare that websites require you to disable.
In the right upper corner in Chrome, you see three vertical dots. Click on that, and then,
1. Click on SETTINGS in the dropdown menu. 2. In the new window, click EXTENSIONS in the left frame (it's at the bottom). 3. Type in AdBlock Plus in the search at the top of the page that shows up after you've clicked. 4. Install. 5. You're done!
Just an FYI if you're frustrated and don't know why your computer is running slow.
EDIT: After you've installed it and want to check the status, click on the icon in the top right corner of the Chrome browser that looks like a puzzle piece. That will open a dropdown menu, and AdBlock will show up there. Also, if a website requires you to disable AdBlock on their page, you click the puzzle piece icon and a text will show up, asking you if you want to disable the blocker on that particular page.