Mac suddenly slow — a 5-minute fix checklist

The Mac was fine all morning. Now it’s lagging — switching apps takes a second, the cursor stutters, audio is glitching. Something changed in the last few minutes, and there’s a fast way to find it.

The five-minute checklist

1. Minute 1 — Activity Monitor, CPU tab. Sort by %CPU. Anything sustained above 50% on one process is the prime suspect. Note its name.
2. Minute 2 — Memory tab. Check memory pressure (the graph at the bottom). Yellow or red means you're swapping, which feels exactly like "slow."
3. Minute 3 — Energy/Disk tab. Sort by disk I/O. If a process is reading or writing 100+ MB/s sustained, it's starving the system.
4. Minute 4 — Thermals. Run sudo powermetrics --samplers smc -i 1000 -n 1. If CPU die temp is above 95C, the chip is throttling — that's why everything feels slow.
5. Minute 5 — Network. If "slow" means "web apps are spinning," run networkQuality in Terminal. A flaky uplink can make the whole Mac feel sluggish even though the Mac is fine.

Most-likely sudden causes

Sudden slowdowns usually have a clear trigger. The big ones:

• A browser tab went rogue (autoplay video, crypto miner, runaway script)
• Spotlight started reindexing after a big file change (mds, mds_stores)
• Time Machine kicked off
• A cloud sync agent started catching up
• Photos started analysing your library (photoanalysisd)
• An app crashed and is in a respawn loop
• Memory pressure crossed a threshold and the system started swapping hard

The fix

Once you know which resource is starved, the action is usually obvious. CPU spike — quit the offending process. Memory pressure — close apps and tabs. Disk thrashing — pause cloud sync or Time Machine. Thermal throttling — pause heavy work, give the chip 60 seconds, check airflow. Network — switch bands or use Ethernet.

Why this beats restarting

Restarting will probably fix it, and you should restart if nothing else works. But restarting also hides the cause. If the same slowdown happens tomorrow, you’ll be back here. Identifying the resource that’s starved teaches you what to watch for, and that saves time in the long run.

Keep the signals visible

You can’t run Activity Monitor and powermetrics continuously. A menubar monitor solves the “by the time I checked, it had passed” problem by keeping every signal live. Beacon surfaces CPU, memory, disk, network, and temperature in the menubar at once — $14.99 lifetime. Download Beacon if you’d like that visibility without the tab-bouncing.