iOS: Open tabs accumulate pretty fast, and on the iPhone, it's hard to tell how many you even have open. The more tabs you have open, the slower your browser is going to move. If you want to close all your tabs at once, iPad Insight points out that you can just enable private browsing.
Music by Thiaz Itch
Hit squares in the bottom right of Safari (or hit the "+" if you're on the iPad) and then tap the "Private" button in the bottom left. You'll see a popup that asks if you want to close or keep all the tabs open; select "Close All." Tap "Private" again to return to normal browsing. It's still a lot of taps, but it's easier than closing each tab individually.