Two very different problems produce the same complaint. A Mac that feels slow might be short on memory, or it might be short on disk. The fixes have nothing in common, and guessing wastes an afternoon, so start by measuring.
Memory pressure, not free memory
Free memory is a misleading number. macOS uses otherwise idle RAM for caching, so a machine with almost no free memory can be perfectly healthy, while a machine with 8GB of free memory can be under pressure from swap.
memory_pressure
vm_stat | head -12The useful signal is memory pressure and swap activity. If swap is several gigabytes and growing while memory free is under 15 percent, the machine is paging to disk. The Status tab in macoptimize tracks both numbers together for exactly this reason, and the screenshot we keep as an example shows swap at 3.4G with memory free at 12 percent, which is a machine that needs fewer open apps, not less data.
What swap really costs
sysctl vm.swapusage
ls -lh /private/var/vmSwap lives on the SSD. When a machine is paging, applications freeze while pages move, and the effect is far more noticeable than a slow disk read. Restarting clears the pressure temporarily. Closing the browser with 60 tabs open clears it properly.
Do not delete swap files. macOS manages them, and removing them from under a running system is how you get a kernel panic.
Thermal throttling
A Mac that is fast for ten minutes and then slow for an hour is usually thermal throttling, not tired. Check the CPU history rather than the current value. A sustained line near 100 percent with the fans audible means the machine is doing work, and the slowdown is the cooling system doing its job.
top -o cpu -n 10If the top process is kernel_task, that is macOS taking CPU time away from applications to cool the machine, which is a symptom rather than a cause.
One runaway process
Most sudden slowdowns have a single cause. Sort by CPU and look at the top entries:
ps aux | sort -rnk 3 | head -10Common offenders on developer machines: an indexing daemon (mdworker_shared), a build that never exited, a simulator that is still booted, a Docker VM that is chewing CPU in the background, or a browser tab running something expensive. In the screenshot of our own Status tab, four separate mdworker_shared processes were the top CPU consumers, all triggered by a large cleanup that forced Spotlight to reindex.
Disk pressure, which is a different problem
df -h /Under 5 percent free space, every write has to find somewhere to go, and you will feel it. This is the case where cleanup genuinely helps performance, and the guides for caches, developer output and System Data all apply.
macoptimize samples CPU per core, memory free, swap and disk continuously and keeps seven days of history. That matters because the interesting question is rarely what the machine is doing right now, it is what changed since last week. The Status tab also lists the top CPU processes with a one click warning banner when swap and memory cross the thresholds you set.
Order of operations when a Mac feels slow
- Check memory pressure and swap. If swap is climbing, close things.
- Check the top CPU process. If one name dominates, deal with it.
- Check free space. If you are under 10 percent, clean up.
- Check uptime. A restart after two weeks resolves a surprising number of things.
- Only then consider hardware. RAM that cannot be upgraded is a real limit, but it is rarely the first explanation.
uptime
sudo purgepurge forces the disk cache to be flushed, which is a safe way to free idle memory before a heavy task. It is not a fix for sustained pressure, it is a way to buy a clean baseline.
Reading memory pressure correctly
Free memory is a misleading number on macOS, because the system deliberately uses idle RAM for caching. The number that matters is pressure:
| Pressure | What it means | What to do |
|---|---|---|
| Green | Plenty of headroom | Nothing, the machine is fine |
| Yellow | Compressing memory, starting to swap | Close the heaviest apps, find what grew |
| Red | Swap is in active use and disk reads are in the path | Quit the offending process, or add RAM |
memory_pressure
vm_stat | head -12
top -l 1 -o mem -n 10In vm_stat, the interesting counters are pageins and pageouts. A pageout means a block of memory was written to the SSD because RAM was needed for something else, and a machine doing that continuously is a machine that will feel slow no matter how fast its processor is.
Finding the runaway process
top -l 1 -o cpu -n 10
ps aux | sort -nrk 3 | head -10Two patterns are worth recognising. A single process pegged near 100 percent is usually a build, an indexer, a stuck backup or a misbehaving Electron app. Two or more processes sharing high load with constant disk activity is usually memory pressure forcing the system to thrash. The first case is fixed by quitting something, the second by closing several things.
Thermal throttling, the silent slowdown
Apple Silicon and Intel Macs both slow themselves down when they get hot, and a laptop on a soft surface in a warm room can lose a surprising amount of speed without any indicator lighting up. Signs worth noticing: fans at maximum during a light workload, sustained performance that drops after ten minutes of work, and a machine that is fast in the morning and slow by afternoon. Elevating the machine and clearing the vents costs nothing and occasionally solves a real performance complaint.
When the disk is the bottleneck
A nearly full SSD behaves very differently from a half full one. APFS needs free blocks to write, and swap needs space that can be reclaimed quickly, so a drive at 97 percent forces the system to work for every write. On a machine that is both slow and nearly full, freeing 30GB often does more for perceived speed than any setting you can change.
The order to work in
- Check memory pressure. If it is yellow or red, deal with memory before anything else.
- Look at the top ten processes by CPU. Quit anything you do not recognise and cannot explain.
- Check free space. Below 10 percent, the disk is contributing to the slowdown.
- Reboot. It clears compressed memory, releases leaked file handles and ends long running background work.
- Check the startup items and login items list, which is where a surprising amount of idle CPU lives.
A reboot is the least fashionable step and still the one that fixes the most machines, because it resets state that nothing else resets.
FAQ
Does clearing disk space make a Mac faster?
It can, and more often than people expect. A drive close to full slows writes, swap and snapshot management, which shows up as general sluggishness rather than as an obvious error. On a machine that is both slow and nearly full, freeing space is the cheapest fix to try first.
Is high swap usage damaging to the SSD?
Modern SSDs are rated for far more writes than normal use produces, and macOS manages endurance itself. Heavy swap is a symptom of insufficient RAM and it will make the machine feel slow, but it is not a realistic threat to the drive on a Mac built in the last decade.
How much memory do I actually need?
For general work, 16GB is comfortable and 8GB is tight if you keep a browser, an editor and a container runtime open together. For development with Docker, Xcode and simulators, 16GB is the practical minimum and 24GB or 32GB removes the problem entirely.
Why is my Mac slow after an update?
Indexing, cache rebuilds and snapshot thinning all run after an update and can take a day or two. Activity Monitor will show Spotlight or a system process using CPU during that time. It usually resolves on its own, and forcing it with a second reboot rarely helps.
Does the purge command help?
It releases inactive memory and can help in the moment, but it is a reset rather than a fix. If pressure is high because a specific app is using 12GB, purging will not change that, and the same state returns within minutes. Use it after quitting the heavy app, not instead.