It seems that using Safari causes my whole system to start lagging.

Can you run EtreCheck and post the report here please?

EtreCheck: https://discussions.apple.com/docs/DOC-6173

When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.

Go step by step by step and test.

1. Power off the router. Unplug it from the wall. Wait a while.

Plug it back to the wall. Power the router on. Wait until all the lights are lit properly. It will take a while.

Restart the computer.

Start up in Safe Mode.

http://support.apple.com/kb/PH14204

2. Delete Caches.db

Close all windows and quit all applications.

Hold "option" key down and click "Go" menu in the Finder menu bar.

Select "Library" from the dropdown.

Library > Caches > com.apple.Safari > Caches.db

Right click the Caches.db file and select "Move To Trash.

Close windows and relaunch Safari.

3. Empty Caches

Safari > Preference > Advanced

Checkmark the box for "Show Develop menu in menu bar".

Develop menu will appear in the Safari menu bar.

Click Develop and select "Empty Caches" from the dropdown.

4. Delete Cookies

Safari > Preferences > Privacy > Cookies and other website data:

Click the “Details” button.

Remove all cookies except ones from Apple, your internet service provider and banks.

5. Safari > Preferences > Extensions

Turn off Extensions if any and launch Safari again to test.

Turn on those one by one and test.