A Mac rarely gets slow all over for no reason. Almost every slowdown traces back to one of a handful of causes, and each leaves a trace you can read in a minute or two with tools already on your Mac. Work through them in this order: the first two cause most slowdowns.
1. Memory pressure is high
When apps need more memory than your Mac has, macOS first compresses what’s in memory, then starts moving it to the disk (called swapping). Swapping is where the beachball comes from.
How to check: open Activity Monitor, choose the Memory tab and look at the Memory Pressure graph at the bottom. Green is fine, however little memory looks free. Yellow means macOS is working hard to make room. Red means it has run out of room and is swapping heavily.
What helps: sort by Memory and quit what’s at the top. Browsers with many tabs are the usual culprit. See what memory pressure means for why the “free” figure is the wrong one to watch.
2. The startup disk is nearly full
Swap lives on your startup disk. When the disk is nearly full, macOS has nowhere to swap to, and memory pressure turns from “a bit slower” into stalls and apps being force-quit. A common rule of thumb is to keep at least a tenth of the disk free.
How to check: Apple menu › System Settings › General › Storage. The bar at the top shows how much is left.
What helps: large downloads and videos you no longer need, and caches that rebuild themselves. If you write code, Xcode alone can hold tens of gigabytes; see whether it’s safe to delete DerivedData.
3. An app is running under Rosetta
On an Apple silicon Mac, an app built only for Intel runs through Rosetta, which translates it as it goes. That costs CPU, memory and launch time. Sometimes an app runs translated even though it contains a native version, because “Open using Rosetta” was ticked once and never unticked.
How to check: in Activity Monitor, choose View › Columns › Kind. Anything marked Intel is running under Rosetta.
What helps: update the app, or select it in Finder, choose Get Info and untick Open using Rosetta if the box is there.
4. One process is using all the CPU
A stuck browser tab, a sync client in a loop or an indexing job can hold a CPU core at full load. After a macOS update, Spotlight (mds_stores) and Photos re-index for a while; that’s normal and finishes on its own.
How to check: Activity Monitor › CPU, sorted by % CPU. Something sitting near 100% for minutes is worth a look. If it’s kernel_task, read what kernel_task’s CPU use means: it’s usually heat.
5. An app keeps growing
Some apps leak memory: they take more and more over hours and never give it back. The Mac is fine in the morning and crawling by the afternoon.
How to check: note an app’s figure in Activity Monitor’s Memory tab, then look again an hour later. Steady growth while you aren’t using it more is the sign. Quitting and reopening the app gets the memory back.
6. Too much starts at login
Updaters and helpers from apps you installed start at login and wake on schedules. They cost login time and occasional bursts of work more than memory.
How to check: System Settings › General › Login Items & Extensions. Turn off what you don’t recognise or need.
What doesn’t help
“Free up RAM” buttons and memory cleaners. They make macOS throw away its cache, the free figure rises for a minute, and your apps read everything back from disk. Here’s why they don’t work.