How to tell if one browser tab is slowing down your Mac

To tell if one browser tab is slowing down your Mac, look at what the browser’s processes are using. Modern browsers run each tab, or group of tabs, as its own process, so a single busy tab shows up as one helper process far above the rest. Find that one, close its tab, and the Mac usually recovers at once, without quitting the browser or losing your other tabs.

Below: why a tab can slow a whole Mac, how to find the busy one, how to match it to a tab in Safari and Chrome, and what to do about it.

Why one tab can slow everything

A web page can run code of its own. It can play video, run a live chart, refresh itself every second, or run a script that has gone wrong and never stops. Whatever it does, it runs on your processor and uses your memory, and a tab you left open in the background keeps doing it.

Browsers keep tabs apart for safety and stability. In Safari each tab, or group of tabs, runs in a web content process. Foreman gives that process the name “Web content” and describes it as rendering a web page for Safari or another app using WebKit. Chrome calls its equivalent a renderer, and on a Mac each one is a process named Google Chrome Helper (Renderer); what Helper (Renderer) processes are covers them in more detail. The point of the split is that a crash in one tab does not take down the others. A side effect is that the cost of each tab is separate, and one tab can use far more than the others.

The usual offenders are:

• pages that autoplay video or animated ads
• web apps left open all day, such as dashboards, chat and mail
• pages with a script stuck in a loop
• pages with many large images or maps
• tabs running video calls, which use the camera and encoding

Find the busy process

The first step is a list of processes sorted by CPU.

1. In Foreman, press Control-Shift-Escape from any app and open the Processes page.
2. Click the CPU column to sort the list, busiest at the top.
3. Find your browser. Its helper processes sit under its row, added up, and the row opens to show them. Foreman gives Safari’s WebKit helpers plain names: Web content (it renders a page), Web networking (it loads web pages and downloads) and Web graphics (it draws web page graphics, video and WebGL).
4. Open the browser’s row. If one helper is far above the rest and stays there for several seconds, that is your tab.

Read the figure over five to ten seconds. A page that is loading spikes and then drops, which is normal. A tab that stays high for a while after it has finished loading is the problem. If you need help reading CPU figures, see how to see which app is using the most CPU on a Mac.

In Terminal, top -o cpu shows the same thing in raw form, with process names but no names of tabs. Press q to leave it.

Also try the Memory column. A tab can be heavy on memory without being busy, such as one holding a huge web app. Memory in a browser is a topic of its own: see why Chrome uses so much memory on a Mac.

Match the process to the tab

Knowing which helper is busy is half the answer. You still need the tab. A process list names processes, not pages, so use these hints, which work whatever list you use.

• Close tabs one at a time, and watch the list. Close the tab you suspect and see whether the helper’s CPU drops or the row disappears. This is slow but sure.
• Look for the tab playing sound or video. Safari and Chrome both mark a tab that is playing audio with a speaker icon. A tab you forgot about, playing in the background, is a common cause.
• Look for pages that update themselves. Live scores, trading charts, dashboards, chat and social feeds update constantly.

When the same page is open in several tabs, close the extras. Duplicates are common and each costs something.

Close the tab or quit the browser

Closing the tab is usually enough, and it costs you less.

Close the tab when you can tell which one it is. Press Command-W. You keep the rest of your tabs, and if you need the page, you can reopen it later from History. If the tab will not close, wait a few seconds; a stuck page can take a moment to respond.

Quit the browser when you cannot tell which tab it is, when many tabs are busy, or when the browser itself is not responding. Press Command-Q. Most browsers offer to restore your tabs next time you open them, though this depends on your settings, so check before you rely on it.

Force quit only when the browser is marked Not responding and will not quit. Open the Force Quit window with Option-Command-Escape, pick the browser and click Force Quit. In Foreman, End Task force quits an app macOS has marked Not responding at once. You lose unsaved changes on pages, such as half-written forms, so copy anything you need first if the browser still lets you.

Do not try to end a single browser helper process directly unless you know what it is. Ending a renderer or Web content process makes the tab or tabs it served crash or reload, and anything typed into them is lost. It can work as a way to stop a runaway tab, but closing the tab is cleaner.

Stop it coming back

A few habits keep tabs from slowing the Mac again.

• Close tabs you are done with. It sounds obvious, but a hundred open tabs is a hundred things the browser may be running.
• Put pages you want to keep into bookmarks or a reading list, not into open tabs.
• Stop autoplay. In Safari, choose Safari > Settings, click Websites, then Auto-Play, and set a site, or all sites, to stop media from playing by itself. Other browsers have their own site settings.
• Keep the browser updated. A runaway tab is often a bug that a browser update has since fixed.
• Be wary of extensions. An extension can run on every page, and a misbehaving one will show as processor use in the browser. If the browser stays busy whatever page is open, try turning extensions off one by one.

For a web app you need open but not right now, such as a mail or chat page, Foreman’s Efficiency Mode can be turned on for the browser. While the browser is in the background it runs at background priority, in short bursts, held to about a quarter of the processor time it would take, and it runs at full speed when it is in front or while it plays sound. It is set per app, so think of it as slowing the browser, not one tab. How much it helps depends on how busy the Mac is.

The short version

One busy helper under the browser means one busy tab. Find it by sorting by CPU and opening the browser’s row, close that tab and leave the browser running. When you cannot find it, quit the browser and reopen it. For the wider picture of what slows a Mac, read why is my Mac so slow.

Foreman is $12 once, free for 24 hours, and needs macOS 14 Sonoma or later.