Most Mac problems can be diagnosed in under a minute with Activity Monitor. The catch is that the interface looks intimidating if you've never used it. This guide walks through it as a friend would, no prior knowledge assumed.
How do you open Activity Monitor on a Mac?
Three ways:
- Spotlight (fastest). Press Command-Space. Type "Activity Monitor". Press Return.
- Through Finder. Open Finder, click Applications in the sidebar, open the Utilities folder, double-click Activity Monitor.
- Launchpad. Open Launchpad (the rocket icon in the Dock or pinch-in on a trackpad), find the Other folder, click Activity Monitor.
Activity Monitor is built into every Mac. You don't need to install it.
What do the five tabs show?
Activity Monitor has five tabs across the top: CPU, Memory, Energy, Disk, Network. They show different aspects of what your Mac is doing.
CPU shows how hard your Mac's processor is working. Each app gets a percentage. The bottom of the tab shows how busy the CPU is overall (System, User, Idle). On a quiet Mac, Idle should be 90% or higher.
Memory shows how RAM is being used. The most useful tab for diagnosing slowness. We'll spend most time here below.
Energy shows what's draining your battery. Only relevant on a laptop. Apps with high "Energy Impact" are the ones to consider quitting if your battery dies fast.
Disk shows what's reading and writing to your hard drive. Useful when your Mac suddenly slows for no reason; it's often a backup or sync app churning the disk.
Network shows what's sending and receiving data over your internet connection. Useful when something feels slow online or when you suspect an app is uploading something it shouldn't.
How do I find what's slowing my Mac?
The fastest diagnosis is in the Memory tab.
- Click the Memory tab at the top.
- Look at the small Memory Pressure graph at the bottom. (See what memory pressure means for the long version.)
- If the graph is mostly green, your Mac has enough memory. Slowness is from something else.
- If it's often yellow or red, click the Memory column header to sort the process list by memory use, biggest first.
- Whatever's at the top is your memory hog. It's almost always Chrome, Slack, Microsoft Teams, Photoshop, or another heavy app.
- If you don't need it running right now, click it once to select, then click the small octagonal stop-sign button in the toolbar (top-left), and choose Force Quit.
That's the diagnostic. The whole flow takes about thirty seconds once you know it.
What if Activity Monitor shows kernel_task using high CPU?
This one confuses people, so it gets its own answer.
kernel_task is the part of macOS itself that manages everything: memory, processes, hardware. You can't quit it (Activity Monitor won't even let you).
When kernel_task uses high CPU (say 100% or more), it's almost always macOS deliberately throttling another app to protect against overheating. The fix isn't to do something to kernel_task; it's to address the heat. Common causes: laptop is on a soft surface (bed, lap), hot environment, charging while doing intensive work.
Move to a hard surface, give it a few minutes, kernel_task will calm down on its own.
What about WindowServer using high memory?
WindowServer is the part of macOS that draws everything on screen. It uses memory in proportion to how many windows you have open and how complex your displays are.
It's normal for WindowServer to use a few hundred MB. It's not normal for it to use 5 GB. If WindowServer climbs over hours of normal use, it usually means a memory leak somewhere; restart your Mac and it goes back to normal.
This was a particular problem in early macOS Sequoia (October 2024 onwards) and again in macOS Tahoe. Apple has shipped patches; keeping macOS up to date helps.
What's safe to force-quit and what isn't?
Safe to force-quit: anything you launched yourself. Apps in your Dock, browsers, chat apps, creative software. Force-quitting them is the same as choosing Quit; you just lose any unsaved work.
Don't force-quit: anything labelled with words like kernel_task, WindowServer, loginwindow, launchd, mDNSResponder, coreaudiod, mds, mds_stores, corespotlightd. These are macOS itself. Force-quitting them can crash the system or break features (Spotlight, audio, network) until you restart.
Rule of thumb: if you don't recognise the name, leave it alone.
How do I know if I have enough memory?
Watch the Memory Pressure graph for a week of normal use.
- Mostly green: you have enough memory. If your Mac feels slow, the cause is elsewhere.
- Often yellow: you're working it. A focused memory tool can extend your Mac's life. (See the best Mac memory cleaners.)
- Often red: you've outgrown your RAM. Time for a new Mac with more.
For Apple's official version of this advice, see Check if your Mac needs more RAM.