Why is my Mac so slow? Seven causes, and how to spot each one

A Mac rarely slows down because it is old. Almost always something specific is competing for the processor, the memory, the disk or the cooling, and macOS shows you which one if you know where to look. This guide covers the seven usual causes in the order worth checking, where each one shows up on macOS 26 Tahoe, and what fixes it. If you already suspect a particular app, the guide on which app is using your CPU or memory reads Activity Monitor column by column.

Find the cause in five minutes

You need Activity Monitor and System Settings, both built into macOS. Nothing to install.

1. Open Activity Monitor. Press ⌘Space, type Activity Monitor and press Return. It also lives in Applications > Utilities.
2. Sort the CPU tab. Click CPU at the top, then click the % CPU column heading so the busiest process is first. Watch the top rows for half a minute.
3. Read the Memory Pressure graph. Click Memory and look at the graph at the bottom of the window. Green is fine. Yellow or red means apps are short of memory.
4. Check free space. Choose Apple menu > System Settings, click General, then Storage. A bar that is nearly full slows everything that writes to disk.
5. Notice heat and timing. A hot Mac with kernel_task near the top of the CPU tab is cooling itself. Slowness that began right after a macOS update is usually indexing.

The table below maps what you find to the fix. Each cause has its own section after it.

The seven causes at a glance

An app that never stops working

This is the easiest cause to spot. In the CPU tab sorted by % CPU, a video export or a code build belongs at the top while it runs. An app you are not using, or a tab you forgot about, does not.

How to read it. The % CPU figure counts one core as 100%, so a single process can show 150% or more, and the scale on an 8-core Mac runs to 800% (The Eclectic Light Company). What matters is a process that stays high for minutes with nothing for it to do.

The fix. Select the process, click the Stop button in the toolbar and choose Quit. Use Force Quit only when Quit does nothing: Apple’s guide warns that if the process has files open, you may lose data (Apple: quit a process). If the app climbs back after a relaunch, look for the reason, for example an update waiting to install, a sync that is stuck, or a plugin.

Memory pressure and swap

Memory pressure is Activity Monitor’s measure of whether memory is short. Apple’s guide says it “is determined by the amount of free memory, swap rate, wired memory, and file cached memory” (Apple: memory usage, macOS 26). A Memory Used figure close to your installed RAM is not, on its own, a problem.

How to read it. Click Memory and look at the graph at the bottom. Apple’s guide puts it plainly: green means the Mac is using all of its RAM efficiently, yellow means it might eventually need more, and red means it needs more RAM (Apple: does your Mac need more RAM). Next to the graph, Swap Used shows how much the Mac is borrowing from the disk. The disk is slower than RAM, so a Mac that keeps moving memory to swap and back spends time waiting on it (Operating Systems: Three Easy Pieces). The glossary explains memory pressure and swap used in more depth.

The fix. Sort the Memory tab by the Memory column and quit the apps at the top you are not using. Relaunch any app whose figure keeps climbing. The full routine, including why RAM cleaners do not help, is in how to free up RAM on a Mac.

A startup disk that is nearly full

Apple lists a startup disk without enough free space as the first thing to check in its own If your Mac runs slowly page for macOS 26. Swap files and caches live on that disk, so when it fills up, memory problems get worse too.

How to see it. Choose Apple menu > System Settings > General > Storage. There is no official threshold, but a bar showing only a few gigabytes free is a clear answer.

The fix. Start with the largest categories in Storage. Apple suggests storing files in iCloud, moving photo and video libraries to an external drive, deleting old iPhone and iPad backups and emptying the Trash, since space is not freed until you do (Apple: free up storage space).

Heat and thermal throttling

Thermal throttling is the Mac slowing its processor to keep temperatures safe. kernel_task is the process Activity Monitor shows for the core of macOS, and this is one of its jobs: Apple says it helps “manage CPU temperature by making the CPU less available to processes that are using it intensely” (Apple: kernel_task and the CPU). So when kernel_task is busy, look for the source of the heat rather than at kernel_task itself.

How to see it. kernel_task near the top of the CPU tab, a warm case and loud fans. In Terminal, this prints any thermal warning macOS has recorded:

pmset -g therm

On a Mac that has not throttled, it answers “No thermal warning level has been recorded”.

The fix. Find the app producing the heat in the CPU tab and quit it. Apple asks you to use a Mac laptop on a stable surface with good ventilation, not on a bed or pillow, at an ambient temperature between 10° and 35° C (Apple: laptop operating temperatures). If the fans are the part that bothers you, see why your Mac’s fans are loud.

Spotlight and Photos indexing after an update

After a macOS update, Spotlight rebuilds its search index and Photos analyses your library again. In the CPU tab this shows up as mds_stores, mds, mdworker_shared, photoanalysisd or mediaanalysisd near the top.

How long it lasts. Apple says that, depending on the amount of data on your device, Spotlight indexing “can take hours or even days”. It finishes sooner when the device is not in use, is connected to power and is online over Wi-Fi or Ethernet (Apple: Spotlight indexing, August 2026). Photos does its people and pets detection while you are not using the app, which, depending on the size of the library, can “take up to a few days” (Apple: People & Pets in Photos).

The fix. Leave the Mac awake and plugged in overnight. Quitting these processes does not help; macOS starts them again and the work resumes.

Login items and background items

Apps that open at login, and helpers allowed to work in the background, use memory and CPU from the moment you sign in. Typical examples are cloud sync clients, updaters and chat apps.

How to see it. Choose Apple menu > System Settings, click General, then Login Items & Extensions. Open at Login lists the apps that start with you. App Background Activity lists apps allowed to run tasks when they are not open, such as checking for updates or syncing data (Apple: Login Items & Extensions, macOS 26).

The fix. Select an item you do not need and click the Remove button. Under App Background Activity, switch off apps you rarely use. That list only lets you turn items off and on, and some switch themselves back on (Eclectic Light: login and background items).

Too many browser tabs

Browsers run tabs and extensions in processes of their own, so a browser with many tabs open adds up to dozens of processes.

On a 16 GB Apple M4 Mac we measured, Chrome alone accounted for 26 of 531 running processes, and one Chrome renderer sat at 171% CPU (snapshot taken October 1, 2026).

How to see it. In Activity Monitor, type the browser’s name in the search field. Chrome also has its own Task Manager: click More, then More tools, then Task Manager, and sort by Memory to see which tab or extension is heavy (Chrome Help: speed up Chrome).

The fix. Close tabs you are not reading with ⌘W and turn off extensions you do not use. In Chrome, Settings > Performance > Memory Saver deactivates tabs you are not using and reloads them when you come back (Chrome Help: Memory Saver).

What does not speed up a Mac

• RAM cleaners. They mostly empty file caches that macOS fills on purpose and would hand back to apps the moment they need it.
• sudo purge. Its own manual page says it empties the disk cache and “does not affect anonymous memory” that apps allocate (purge(8)).
• Reinstalling macOS first. Without knowing the cause, the same app or setting slows the fresh system down again.

A restart does help when an app’s memory has grown or a process is stuck, because it starts everything clean.

Where Orcylo fits

Activity Monitor shows the present moment. When a Mac is slow on and off, the useful question is what was busy ten minutes ago, or yesterday. We make Orcylo, a menu bar system monitor for macOS 15 and later that groups the hundreds of processes on a Mac under the apps that own them and keeps 30 days of per-minute history. It notifies you when an app burns CPU for minutes, when its memory keeps growing, when memory pressure turns critical or when the disk fills up. See what Orcylo does, how it compares with Activity Monitor itself, or the other Activity Monitor alternatives we ranked.

How we checked

Menu paths and Activity Monitor labels were checked against Apple’s macOS 26 Tahoe user guides on October 1, 2026. The Terminal commands and the process counts above were measured the same day on a 16 GB Apple M4 Mac with 10 cores. Each claim credited to Apple or Google went through a fact check against the page it links to.