Why Is My Mac So Slow? A Checklist to Find the Cause

A slow Mac usually has one of four causes: an app using too much processor time, not enough memory, a nearly full startup disk, or heat that makes the Mac slow itself down. Less often it's too many apps opening at login, an outdated macOS or a browser extension. Find out which one it is first, then fix that one thing.

Start with a restart

If the Mac hasn't restarted in weeks, choose Apple menu › Restart. That clears stuck processes and swap files. If the Mac feels fast again, keep an eye on it. If the slowness comes back, work through the checklist below. Each item says how to check it in Activity Monitor, the Mac's built-in task manager, and in SystemWatch.

The checklist

1. An app hogging the processor

Signs: loud fans, a warm MacBook, lag everywhere, fast battery drain.

Check in Activity Monitor: open the CPU tab and click % CPU to sort. 100% equals one full core. If one process stays near the top for minutes while you aren't using it, that's your cause. Quit it, or force quit it if it won't respond. On Apple silicon, also check the Kind column: apps marked "Intel" run through Rosetta and use more processor time. Look for an Apple silicon version.

Check in SystemWatch: the Activity view sorts by impact and explains what each process is, and an alert tells you when an app has been working hard for minutes. macOS processes have their own pages, such as mds_stores and WindowServer.

2. Memory pressure and swap

Signs: switching apps takes seconds, the spinning cursor shows up often, browser tabs reload when you go back to them.

Check in Activity Monitor: open the Memory tab and look at the Memory Pressure graph at the bottom. Green is fine, even if Memory Used looks nearly full, because macOS uses spare memory to cache files. Yellow means it's compressing memory. Red, with Swap Used growing, means the Mac is moving memory to disk, and that's slow. Sort by Memory to see which apps hold the most. An app whose memory keeps growing for hours may be leaking it; restart that app.

Fix: quit apps you aren't using, close browser tabs and restart leaky apps. If pressure turns red with your usual set of apps, the Mac needs more memory than it has for that work.

Check in SystemWatch: memory pressure and swap are on the Overview, with history. You get an alert when memory runs low or one app uses a lot.

3. A nearly full disk

Signs: everything is slow, apps fail to save, macOS updates won't install.

Check: choose System Settings › General › Storage. macOS needs free space for swap, caches and updates. As a rule of thumb, keep at least 10% of the disk or 20 GB free, whichever is more.

Fix: empty the Trash, delete large files you don't need, and remove what's safe in System Data. See how to clear System Data and how to clear cache.

Check in SystemWatch: the Storage scan shows what takes the space and explains each item, and an alert warns you when the disk is almost full.

4. Heat and thermal throttling

Signs: the Mac is fast at first and slows during long jobs, it's hot, the fans are loud.

Check: macOS has no temperature readout. In Terminal, sudo powermetrics --samplers thermal -n 1 shows the thermal pressure level. Anything other than Nominal means macOS is working to cool down, and at higher levels it limits speed.

Fix: give the Mac airflow and close heavy apps. See MacBook overheating for the full list.

Check in SystemWatch: the website edition shows every temperature sensor, thermal pressure and core frequencies, so you can see throttling as it happens.

Signs: slow for the first few minutes after you log in, a crowded menu bar.

Check: choose System Settings › General › Login Items & Extensions. Remove apps you don't need from Open at Login, and switch off background items you don't use. How to stop apps opening at startup has the details.

Check in SystemWatch: the website edition's Startup Items view lists each launch agent and daemon with the app it belongs to.

6. Outdated macOS or apps

Check: System Settings › General › Software Update, and Updates in the App Store app. Performance bugs are often fixed in updates. Update apps you installed from websites too.

Right after a major update, the Mac can feel slow for a few hours while Spotlight and Photos rebuild their indexes (mdworker_shared, photoanalysisd). Leave it plugged in and let that finish.

7. Browser extensions and tabs

Signs: the Mac is slow mainly while the browser is open.

Check: in Safari, choose Safari › Settings › Extensions and turn off extensions you don't use. In Chrome, go to chrome://extensions. Chrome's Window › Task Manager shows which tab or extension uses the most memory and CPU. Try working with fewer tabs open for a day.

8. Low Power Mode and syncing

Two things slow a Mac down on purpose or for a while. Low Power Mode (System Settings › Battery) reduces performance to save energy. iCloud Drive syncing a large folder can keep cloudd and fileproviderd busy for hours. Both are normal, but worth knowing about.

What won't speed up your Mac

"Memory cleaners." macOS manages memory well. Forcing it to drop cached data just means it reads that data back from disk.
Clearing caches regularly. Caches make apps faster. Clearing them frees space, but apps run slower until they rebuild.
Deleting system files. It can break macOS. Remove only items you understand.

See it all in one place

SystemWatch keeps the numbers from this checklist in one window: CPU, memory pressure and swap, disk space, network and battery, with 24 hours of history so you can see when a slowdown started. Each process has a plain-language explanation. The website edition adds temperatures, Startup Items, and per-app disk and network use.