App store and safari are very slow and sometimes doesn't load

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.

• Restart the router and the broadband device, if they're separate. Any change?
• Run Software Update and install all available updates for OS X or the computer's firmware. Any change?
• If possible, connect to the router with an Ethernet cable and turn off Wi-Fi. Any difference?
• Are any other devices on the network, and if so, are they the same?
• If you can connect to more than one network, are they all the same?
• Are all network applications affected, or only some? If only some, which ones?
• Is networking always the same, or is the problem intermittent?
• Disconnect all other devices from the network. Any change?
• If you're running a MacBook with the lid closed, open the lid. Any difference?
• Start up in safe mode and test. Any difference?
• Start up in Recovery mode. From the OS X Utilities screen, select Get Help Online. A clean copy of Safari will launch. No plugins, such as Flash, will be available. Any difference?
• If possible, turn off Bluetooth and disconnect any USB 3 devices. Any difference?

Back up all data.

Unlock the Network preference pane, if necessary, by clicking the lock icon in the lower left corner and entering your password. Cllck Advanced, open the DNS tab, and change the server addresses to the following:

8.8.8.8

8.8.4.4

That's Google DNS. Click OK, then Apply.

In Safari, select

Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data

and confirm. If you’re using another browser, empty the cache. Test. Any difference?

Notes:

1. If you lose Internet access after making the above change to your network settings, delete the Google servers in the Network preference pane, then select the TCP/IP tab and click Renew DHCP Lease. That should restore the original DNS settings; otherwise restore them yourself. Remember that you must click Apply in order for any changes to take effect.

2. I don't use Google DNS myself, though I have tested it, and I'm not recommending it or any other DNS provider; the server addresses are offered merely for testing purposes. There may be privacy and technical issues involved in using that service, which you should investigate personally before you decide whether to keep the settings. Other public DNS services exist.

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of this test is to determine whether the problem is localized to your user account. 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 in OS X 10.7 or later, 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.

It's not really clear whether you still had problems with Safari while in Recovery mode. If you did, the cause is external to the computer.

While in Recovery mode, do pages render as in the screenshots? If the answer is "no," see below. If the answer is "yes," the problem is in the network. Maybe you need a new router.

If you don't already have a current backup, back up all data. There are ways to back up a computer that isn't fully functional. Ask if you need guidance. You can skip this step if you don't care about the data that hasn't been backed up.

Reinstall the OS.* Your data should be unaffected unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

If you use FileVault 2, then before running the Installer you must launch Disk Utility and select the icon of the FileVault startup volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another icon with the same name. Click the Unlock button in the toolbar and enter your login password when prompted. Then quit Disk Utility to be returned to the main Recovery screen.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.9 ("Mavericks"), but the procedure is the same for OS X 10.7 ("Lion") and later.

From the menu bar, select

 ▹ System Preferences ▹ Network

If the Network preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Then click the Advanced button and select the Hardware tab. Select

Configure: Manually

and

MTU: Custom

Enter an MTU value of 1280.

Click OK and then Apply. Test.

You must apply the change before it takes effect.

If the issue is resolved, experiment with higher MTU values until you find the highest one that works. The higher the MTU, the better the network interface will perform.

If changing the MTU has no effect, restore the Configure setting to Automatically.

FWIW - I was having the same problem and I just followed Linc Davis's advice about setting MTU to Custom under Sys Pref. I started out at 1280 and it cleared it up. I increased it to 1400 - same. Then to 1480 - same, i.e. good. Then to 1500, which is the same value under the Automatic Configuration and it was still much very good. I am going to leave it as a manual custom configuration at 1500 since it works. BTW - I recently reloaded my entire OS on my MacBook Pro but I am not sure if that had anything to do with it. It was slow before - massively slow - before I just did the latest. Thanks Linc.