Restart it properly

Closing the lid puts your Mac to sleep. It does not clear anything out. Click the Apple menu, then Restart, and let it come all the way back.

If your Mac has been awake for weeks, start here. It is the single most effective step on this list.

Trim your login items

Apps that open when you log in compete for memory before you touch anything.

• Open the Apple menu, then System Settings.
• Click General, then Login Items.
• Under Open at Login, select anything you do not need and click the minus button.
• Look at Allow in the Background underneath. Turn off entries belonging to software you no longer use.

Removing a login item does not delete the app. You can still open it yourself.

Find out what is actually using the machine

Open Activity Monitor from Applications, then Utilities. Click the CPU tab and sort by the percentage column.

One app sitting at the top for minutes at a time is your answer. Quit it and see whether the Mac recovers. Browser tabs are a common cause, so check your browser first.

Make room on the drive

Open the Apple menu, then System Settings, then General, then Storage. macOS shows you exactly what is taking up space.

• Empty the Trash.
• Check the Documents and Applications sections for large files you forgot about.
• Move photo and video libraries to an external drive if the drive is close to full.

Leave yourself some headroom. A drive that is nearly full slows the whole system down.

Install updates

Open System Settings, then General, then Software Update. Install what is offered and restart.

Update your apps as well. Open the App Store and click Updates in the sidebar.

When the problem is the hardware

Many older Macs shipped with 8 gigabytes of memory, and a modern browser can use most of that on its own. Open Activity Monitor and click the Memory tab. Watch the Memory Pressure graph at the bottom while you work normally. If it stays yellow or red, your Mac needs more memory than it has.

On recent Macs the memory is built into the chip and cannot be added later. That is worth knowing before you choose your next one, because the amount you pick on the day is the amount you keep.

Still stuck?

If your Mac got slow suddenly, or it only struggles at certain times, the speed is a symptom rather than the problem. That is worth looking at properly before you spend money on hardware.