Mac running slow? How to get the speed back again

A Mac that has gone sluggish usually comes down to a nearly full drive, apps that launch themselves at login, or a machine with less memory than what you are asking it to do. The first steps take about fifteen minutes and are enough on most Macs.

You do not need to install anything. Everything below is already part of macOS.

Try this first

1. Restart. Click the Apple symbol at the top left and choose Restart. Many Macs never get restarted because the lid simply gets closed, which leaves everything sitting in memory week after week.
2. Check how much space is left. Apple menu, System Settings, General, Storage. A coloured bar appears at the top. If less than ten per cent is free, that is almost always the whole explanation.
3. Empty the bin. Hold your finger on the bin at the end of the Dock and choose Empty Bin. The space is only released at that point.
4. Clear out Downloads. Open Finder and click Downloads in the list on the left. Switch to list view and click the Size column to sort. There are often several gigabytes of old installer files sitting there that nobody needs.
5. Use Apple's own suggestions. On the same Storage page there is a Recommendations section. Turn on Empty Bin Automatically, and click Review Files to see the largest items on the computer gathered in one place.
6. Restart and see how it feels. Open what you normally use. If the drive got some breathing room, you notice straight away.

If that did not help

1. Stop apps launching at login. System Settings, General, Login Items and Extensions. Under Open at Login are the apps that start on their own. Select one you do not need immediately and click the minus button. The app stays installed and opens as usual.
2. Look at the list under the same heading. Below are background extensions belonging to apps you have installed. Switch off anything belonging to an app you no longer use.
3. See what is keeping the Mac busy. Open Launchpad, click the Other folder and choose Activity Monitor. Click the CPU tab, then the % CPU column so the busiest item moves to the top. If something stays high for several minutes, write the name down. To close it, select the row and click the cross at the top left.
4. Check the memory. In Activity Monitor click Memory. At the bottom is a graph called Memory Pressure. Green means the Mac has room. Yellow or red all the time means the memory is not enough for what you have open, and having fewer apps running at once helps directly.
5. Close tabs. Every tab in Safari or Chrome takes memory. Thirty tabs weigh as much as one large application.
6. Update macOS. System Settings, General, Software Update. Newer versions carry fixes that make the machine steadier. If your Mac is older, read the section below first.

On an older Mac

If the computer is seven years old or more, two things are worth knowing. The newest version of macOS your Mac accepts is not always the one that runs best on it, and a Mac from before 2017 usually has memory soldered to the board, so it cannot be expanded.

Before you reinstall macOS or erase anything: back up with Time Machine to an external drive, and make sure you know the password for your Apple Account. Activation Lock asks for it when the Mac restarts after an erase, and without that password the computer cannot be used again. Always type the password in yourself and never give it to anyone, including us.

If the fan runs hard all the time even though the drive has space, it is usually dust in the fans or one app working away in the background. The name you noted in Activity Monitor is the thing to look into.