1. Back up

Before you start tweaking, back up all of your important data, including your music, videos, emails, photos, apps, documents and system files. This way, if you accidentally delete something vital, you can easily get it back.

You can use the Mac’s own built-in Time Machine to do this and back up to an external drive. Alternatively, you can also use iCloud, or back up to DVDs/CDs.

2. Investigate possible causes

Background processes and apps take up processing power and memory space.  If you want to find out exactly what’s eating up your system resources, there’s a handy feature to help. The Activity Monitor tool will tell you what’s running on your Mac, how much memory and processing capacity they’re taking up, and how much space is available on the hard disk.

Open Applications, then Utilities, then Activity Monitor.

You can find out what’s hogging processing resources via the CPU tab, and memory via the Memory tab, while the Disk tab shows how much free disk space you have.

• Clicking the CPU tab will show a list of items, the ones at the top being those that suck up the most processing power.

3. Check your disk space

A lack of disk space will slow down any computer. The Mac’s operating system and apps generate temporary cache files and take up memory while swapping things between the disk and the computer’s memory. These activities take longer if your disk is full, because files need to be deleted before new ones are created.

One way to check available hard disk space is to select About This Mac in the Apple menu. Click More Info and then the Storage tab.

4. Delete files you don’t need

If you’re anything like me, your Downloads folder is a good place to start. Mine gets packed with stuff I’ve saved over time that I then forget about.

Next, take a look at what’s lurking in your Desktop and Documents folders. Don’t forget to look through your photos, videos and music. And if you’re deleting from iPhotos, empty the iPhotos Trash can.

• Move data you want to store elsewhere to an external storage device.
• Compress files that you don’t often use, and delete the original.
• Delete items by dragging them to the Trash. Then empty the Trash.

5. Zap unnecessary apps

Unused apps still use storage space, so having a lot of apps open puts an unnecessary load on your machine. Older Mac models might suffer as a result.

• Quit open apps that you’re not using. Closing windows doesn’t remove them from memory, so the app remains running in the background. To do this, right-click on any Dock icons for apps you don’t need at the time, and select Quit. You can tell which apps are open by the telltale light beneath their icons in the Dock.
• Delete applications by dragging their icon from the Applications folder to Trash. (Note that this may still leave behind files associated with applications).
• Reboot when you’ve finished tidying up as this will clear temporary caches and give you more disk space.

6. Give your hard drive some TLC

Your Mac has an in-built tool called Disk Utility. Open this program and select the hard drive in the panel on the left. In the main panel, click on Verify Disk. The program will begin checking your hard drive.

If the program reports errors in the disk, go to the First Aid tab and select Repair Disk. The Disk Utility program will try to repair any errors.

7. Boost boot-up

Mac booting up slowly? This may be due to having too many applications set to open by default on start-up. Log-in items can be handy, or a hindrance. If they’re the latter, get rid of them.

To check the apps that are set to load automatically, open System Preferences, click on Users & Groups and then on Login Items. Uncheck any app that you don’t need to launch on start-up, then click on the ‘minus’ button.

• Disable unwanted widgets – such as the calculator or stocks – in the Notifications Center. Under System Preferences, select Notifications and then Today. Uncheck all those that you don’t need.

8. Update your operating system and app software

Old operating systems generally run more slowly, so installing the latest version of your Mac’s OS X should give a noticeable boost to performance. It will also ensure you have the latest app and security updates, patches and performance improvements. The latest version available is OS X Yosemite.

Your Mac should notify you when updates become available, and you can also check manually for available updates. Click here for details on Apple's support page.

9. Install and run Internet security software

Although Macs are less prone than Windows PCs to viruses and other malware, they’re not immune. Malware can significantly affect your computer’s performance, so don’t take the risk.

• Install comprehensive antivirus software (free trial of antivirus for Mac) and run regular scans.
• Set the software to update automatically to ensure maximum protection from the latest threats.

10. Declutter your desktop

If you can hardly see your screen for icons, you’re not doing your Mac any favours.

Your Mac’s OS X treats each desktop item as a discrete window, which takes up its own memory. Combining files into folders will minimise both desktop overcrowding and memory use. To do this, drag the files you want to move into folders on your desktop, or move them to the Documents folder. If you no longer want the file, move it to the Trash. Then empty the Trash.

• To avoid this clutter in the future, stop saving things to your desktop.

11. Install more RAM 

Random Access Memory lets programs rapidly access stored information. Running out of RAM can therefore significantly affect system speed, leaving your Mac to rely on sluggish virtual memory.

Check the amount and speed of the RAM you have installed by selecting About This Mac from the Apple menu.

Depending on the Mac model you have, you may be able to add more RAM. If you can, be sure to find out the type and speed of RAM for your model.