Safari

Try this:

(Pre-Sierra)

Force Quit Safari ( command + option + esc keys). Then restart Safari holding the Shift key.

Turn off Safari extensions in Safari > Preferences > Extensions > un-checkmark all extensions.

Empty Caches. You can either choose Safari menu > Clear History Or, if you do not want to remove History go to Safari menu > Preferences > Advanced and check the “Show Develop Menu” in Menubar. Then, on that menu > “Empty Caches”.

See these Apple Support Topics for detailed advice:

If Safari is slow, stops responding, quits unexpectedly, or has other issues - Apple Support

If Safari doesn't load a page or webpage items are missing - Apple Support

Safari for Mac: If Safari can’t open a website

Quit Safari then from your Finder "Go" menu > Go to folder and copy & paste this path:

~/Library/Preferences/com.apple.Safari.plist.

Press Return. Then drag the plist to your Desktop.

Restart Safari and test.