Please answer as many of the following questions as you can. You may already have answered some of them. In that case, there's no need to repeat the answers.
Back up all data before making any changes.
Have you restarted your router and your broadband device (if they're separate) since you first noticed the problem? If not, do that now and see whether there's any change.
If your browser is Safari, then from the Safari menu bar, select
Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data
and confirm. If the Downloads button (with the icon of a downward-pointing arrow) is showing in the toolbar, click it and then click Clear in the box that appears. The download history will be removed. Any change?
If you're running OS X 10.9 or later, select the Advanced tab in the Preferences window and uncheck the box marked
Stop plug-ins to save power
Any change?
Quit and relaunch the browser. Any change?
Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your documents or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this behavior; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. Same problem?
After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
Are any other web browsers installed, and are they the same? What about other Internet applications, such as iTunes and the App Store?
If other browsers and Internet applications are also affected, follow these instructions and test. Any change?
If Parental Controls is active for any user, please turn it off and test. Any change?
If only Safari is affected, launch the Activity Monitor application and enter "web" (without the quotes) in the search box. If a process named "Safari Web Content" is shown in red or is using more than about 5% of a CPU, select it and force it to quit by clicking the X or Quit Process button in the toolbar of the window. There may be more than one such process. Any improvement?
Follow the instructions in this support article. Any change?
Open the iCloud preference pane and uncheck the box marked Photos, if it's checked. Any change?
Are there any other devices on the same network that can browse the Web, and are they affected?
If you can test Safari on another network, is it the same there?
If you connect to your router with Wi-Fi and you can also connect with Ethernet, do that and turn off Wi-Fi. Any difference?
