• Safari is not slowed by tabs in the least. The number of tabs you have open has nothing to do with how quickly Safari operates for you.

• Safari on the iPhone has a maximum of 500 active tabs set by Apple. Consider yourself fortunate if you were unaware of this. It indicates you're probably only using a few tabs, which makes things easier for you. However, if you're approaching or have already surpassed that limit (which isn't ideal), you're in desperate need of assistance.
• It's simple to launch a new tab, but it's not always straightforward to see how many tabs you have open or to fast close them all (and they can add up over time). That can deplete your battery quickly.

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