Why is my Mac slow? A diagnostic checklist

Your Mac feels sluggish — app launches drag, switching windows stutters, scrolling isn’t quite buttery. Before reinstalling macOS or buying RAM you don’t need, run a short diagnostic. “Slow” is almost always one of five things, and you can rule each out in a few minutes.

The five suspects

Every Mac slowdown traces back to one (or more) of these:

• A process pegging CPU
• High memory pressure forcing the system to swap
• A disk that’s almost full or doing heavy I/O
• Thermal throttling because the chip is too hot
• A saturated or flaky network connection making apps appear to hang

The trick is checking them in the right order.

Walk through the checklist

1. Check CPU first. Open Activity Monitor, click the CPU tab, sort by %CPU. Anything sustained above 50% on a single process is a red flag — Spotlight indexing (mds_stores), a runaway browser tab, or a misbehaving background helper are the usual culprits.
2. Check memory pressure. Same window, Memory tab. If the pressure graph is yellow or red, the system is paging out to disk, which feels exactly like "slow."
3. Check disk usage. Open the Disk tab. Sustained reads or writes over 100 MB/s from a process you don't expect (Time Machine, a cloud sync agent) will starve everything else.
4. Check thermals. Run sudo powermetrics --samplers smc -i 1000 -n 1 in Terminal — if CPU die temperature is above 95C, the chip is throttling.
5. Check the network. If "slow" really means "web apps hang," run networkQuality in Terminal to see uplink/downlink.

The Activity Monitor problem

Activity Monitor works, but you’re bouncing between five tabs to triage one slowdown. By the time you’ve checked CPU, the process that was spiking may have settled. There’s no shared timeline, no thermal data, and no fan speeds.

A menubar monitor solves this by keeping every signal live and visible. Beacon shows CPU, memory pressure, disk I/O, network throughput, and temperature in the menubar simultaneously — so when the Mac stutters, you glance up and the culprit is already on screen. It’s $14.99 once. Download Beacon if you’d rather not chase tabs.

After you find the cause

The fix depends on the suspect. CPU spike from a known app — quit and relaunch it. Memory pressure — close tabs, quit Electron apps, restart memory-leaky processes. Thermal throttling — improve airflow, check for blocked vents, or stop sustained heavy workloads. Disk pressure — pause cloud sync, check for runaway logs in /var/log. Network — switch from Wi-Fi to Ethernet or a different band.

The point of the checklist isn’t to fix every Mac problem; it’s to stop guessing. Once you’ve identified which resource is starved, the fix is usually obvious.