Why Your Mac is Slow
A slow Mac usually comes down to one of four things: not enough free storage (macOS needs 10-15% free to run smoothly), too many startup items loading at boot, runaway processes consuming CPU in the background, or accumulated cache data slowing down app launches.
Let's fix each one.
1. Check Available Storage (The #1 Cause)
When your Mac has less than 10-15% free storage, everything slows down. macOS uses free disk space for virtual memory, caching, and system operations. Below 10%, you'll notice significant lag.
If you're below 20 GB free on a 256 GB drive (or below 50 GB on a 512 GB drive), storage is likely your bottleneck. See our guide to freeing up space on Mac for detailed cleanup steps.
2. Find Resource-Hungry Processes
Open Activity Monitor (search for it in Spotlight with Cmd+Space) and click the CPU tab. Sort by "% CPU" descending. Anything consistently above 100% is a problem.
Common culprits after an update:
- mds_stores / mdworker: Spotlight is re-indexing. This is normal after an update and resolves itself in 1-24 hours. Let it finish.
- photolibraryd: Photos is re-analyzing your library for face recognition. Can take hours with large libraries.
- kernel_task: macOS throttling CPU to manage heat. Usually resolves after indexing finishes.
- WindowServer: High usage here means too many windows, displays, or transparency effects.
If a specific app is eating CPU and you don't need it, force quit it (Cmd+Option+Esc).
3. Reduce Startup Items
Too many apps launching at login slows down boot time and eats RAM. Check what's loading:
System Settings > General > Login Items
Remove anything you don't need launching at startup. Also check LaunchAgents for hidden startup items:
You'll often find old agents from apps you uninstalled long ago, still running in the background and using resources.
4. Run Maintenance Scripts
macOS has built-in maintenance scripts that clear logs, rebuild databases, and clean up system caches. They're supposed to run automatically but sometimes don't (especially on laptops that are asleep at night).
This takes 1-2 minutes and can noticeably improve performance by clearing accumulated log files and rotating databases.
5. Rebuild Spotlight Index
A corrupted Spotlight index can cause constant CPU usage and slow searches. Force a fresh index:
Spotlight will re-index in the background over the next few hours. Your Mac might feel slower during indexing but will be faster once it's done.
6. Clear DNS Cache
If your Mac is slow specifically with internet browsing (pages take forever to load), clearing the DNS cache often helps:
This forces your Mac to do fresh DNS lookups instead of relying on potentially stale cached data.
7. Reduce Transparency and Motion Effects
If your Mac is older or has integrated graphics, the macOS visual effects (transparency, window animations, blur effects) can consume significant GPU resources.
System Settings > Accessibility > Display > check "Reduce transparency" and "Reduce motion"
This makes a noticeable difference on MacBook Airs and older Macs, especially when using multiple desktops or lots of windows.
8. Clear Accumulated Caches
Over time, app caches can grow to 10-20 GB and slow down both app launches and disk I/O. Clearing them forces apps to rebuild fresh, smaller caches.
If it's over 5 GB, clearing it will help. You can do this manually (rm -rf ~/Library/Caches/*) or use a cleanup tool that classifies which caches are safe to delete and which aren't.
Not all caches are equal. Browser caches are safe to clear. But deleting AudioUnitCache (if you use Logic Pro) triggers a 30-minute plugin re-validation. Deleting IDE caches (Xcode DerivedData) means rebuilding on next compile. A good cleaner tells you the risk level of each item before you delete it.
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When to Consider a Clean Install
If your Mac is still slow after all these steps, especially after a major macOS upgrade (e.g. jumping from Monterey to Sonoma), a clean install might be your best option. The upgrade process carries forward old system files, caches, and preferences that can conflict with the new OS.
Back up everything to Time Machine or an external drive, then boot from a macOS installer USB and do a fresh install. This is the nuclear option but it works.
Quick Summary
- Check storage first: below 10-15% free = slow Mac
- Activity Monitor shows you exactly what's eating CPU
- Remove unnecessary startup items (Login Items + LaunchAgents)
- Run maintenance scripts: sudo periodic daily weekly monthly
- Clear caches, but be careful with audio/dev caches
- After major updates: give Spotlight 1-24 hours to finish re-indexing