New windows and tabs in Safari by default display a 3×4 grid of Top Sites, which shows the web sites you visit the most often. Sometimes it will display sites you don’t want showing up, and it can also slow down Safari on older computers. If you would like to possibly speed up Safari and avoid any potential embarrassment try disabling Top Sites from appearing in new windows and tabs:
Open Safari menu and go-to Preferences (Command + ,).
Under the “General” tab look for “New windows open with:” and choose any option other than Top Sites.
Below that you'll find “New tabs open with:” and choose “Empty Page” or again any option other than Top Sites.
Open a new tab or window to confirm the change. You can always get Top Sites back by selecting it again in the same preference panel.