The reassuring part first: sudden slowdowns almost never mean your Mac is failing. In almost every case, something changed in the last day or two - software-side - and once you find it, the fix is straightforward. This guide works through the four most common causes in order of likelihood, then tells you exactly what to check and in what order.
The four most common causes of sudden slowdown
When a Mac goes from fine to sluggish with no obvious trigger, the cause is almost always one of these four things.
A macOS update ran in the background. After any system update, macOS kicks off indexing, re-analysis, and recompilation tasks that can run for hours. The process called mds_stores handles Spotlight indexing; photoanalysisd re-scans your Photos library. Both use significant CPU and can make the whole machine feel sluggish until they finish.
A new app installed a background helper. Many apps - especially utilities, backup tools, and browser extensions - add processes that run at all times, not just when you open the app. If you installed something new in the last week, its helper process might be running even now, quietly consuming CPU or memory.
Memory pressure built up over the day. RAM (the temporary workspace macOS uses to hold everything that's running) fills up gradually as apps accumulate data. A browser with thirty tabs, a communication app like Slack, and a few other apps left open since morning can collectively leave very little RAM free. When that happens, macOS starts writing data to disk as overflow - a process called swapping - and the Mac slows noticeably. See what memory pressure means on Mac for a detailed explanation.
Storage is nearly full. macOS needs free disk space to use as swap. If your drive has less than roughly 5-10% free, the swap system breaks down and slowdowns follow. Check Apple menu › About This Mac › More Info › Storage.
Did you update macOS recently? (the 72-hour rule)
If your Mac slowed down within a day or two of a macOS update, that update is almost certainly the explanation. This is so common it deserves its own name: the 72-hour rule. Give it three days. The background tasks macOS runs after an update - indexing, recompilation, re-analysis - are finite. They stop when they're done, and your Mac returns to normal.
You can check whether indexing is still running by opening Activity Monitor and searching for mds_stores. If it shows high CPU usage, indexing is in progress. You can't safely stop it, but you can confirm it's the cause.
If you're on macOS Sequoia or Tahoe and the slowdown started right after the update, there's more detail in the dedicated post on why your Mac is slow after a macOS update. Apple also patches performance regressions in point releases (like 15.1, 15.2), so keeping your software current helps.
One thing worth checking: open System Settings › General › Software Update. If an update downloaded overnight but not yet installed, it may be sitting in a partially-applied state. Finishing the installation and restarting often resolves the issue immediately.
What you installed in the last week
New software is the second most common trigger. Many apps that seem self-contained actually install persistent background components: login items that start at boot, helper daemons that run constantly, or browser extensions that intercept every page load.
To see what's running at login, go to System Settings › General › Login Items & Extensions. Anything listed there starts automatically when you log in and stays running. If something appeared there recently that you don't recognise, it may be the cause. You can toggle items off and restart to test.
Browser extensions deserve particular attention. A poorly written extension can slow page loads across all tabs, increase the browser's memory use considerably, and run background scripts even when no tab is open. If you've installed a new extension recently, try disabling it in your browser's extension manager and see whether the slowdown improves.
To check recently installed apps on a Mac, open Finder › Applications, then sort by Date Added. Anything from the last week is worth reviewing. If removing or disabling it stops the slowdown, you've found your answer.
The "open all day" memory creep
Some slowdowns aren't caused by anything new. They're caused by apps that have been running since morning, accumulating memory as the day goes on. This is especially common with Chrome (which gives each tab its own process), Slack (which loads a full web app into memory), and any app with a known memory leak.
A memory leak (jargon explained: a bug where an app continuously uses more RAM without releasing it) means an app that was using 200 MB at 9 AM might be using 1.5 GB by 3 PM. The app doesn't crash; it just quietly grows until it crowds out everything else.
The result is what Activity Monitor calls memory pressure (the measure of how hard macOS is working to keep all running apps supplied with RAM). When pressure turns yellow, the Mac slows. When it turns red, it can become almost unusable.
If your Mac is fine after a restart but slows down by mid-afternoon every day, a leaking app is very likely the cause. Open Activity Monitor in the afternoon when things feel sluggish, click the Memory tab, and sort by the Memory column. The top entry is your suspect. Quitting and relaunching that app often recovers hundreds of megabytes immediately.
What to check first, in order
Rather than guessing, run through this sequence. It takes five minutes and will identify the cause in the majority of cases.
1. Open Activity Monitor. Press Command-Space, type "Activity Monitor", press Return. This is the built-in tool that shows exactly what every process on your Mac is doing. Apple's official Activity Monitor guide explains each column in detail. For a more practical walkthrough, see how to use Activity Monitor on Mac.
2. Check the CPU tab first. Sort by the % CPU column by clicking the header. If any single process is using more than 80% of CPU and you don't recognise it, that's your cause. Common culprits after an update: mds_stores (Spotlight indexing) and photoanalysisd (Photos re-analysis). Both are benign and temporary.
3. Check the Memory tab. Click the Memory tab and look at the Memory Pressure graph at the bottom of the window. If it's green, RAM isn't the issue. Yellow means the system is under strain. Red means it's critical - sort the list by the Memory column to find the biggest consumer and consider quitting it.
4. Check swap usage. Still in the Memory tab, look for the "Swap Used" figure near the bottom. If it reads several gigabytes, your Mac is running out of RAM and using disk as overflow. Quitting apps and freeing memory is the immediate fix. A restart is the fastest reset.
5. Restart if nothing is obvious. A restart clears accumulated memory use, kills runaway processes, and resets the swap file. If the Mac returns to normal after a restart and then slows again over the following hours, you now know it's a cumulative memory problem. Check login items and consider whether an app is leaking.
If the slowdown returned immediately after a restart and it didn't exist last week, look hard at what changed between then and now: any macOS updates, any new apps, any new browser extensions. The answer is usually there.
For a broader look at general Mac slowness rather than sudden-onset cases, the guide on why your Mac is so slow covers the full picture.