The reason for the slowness of your Mac computer and the methods to improve the performance is explained below.

1-Desktop Excessively Cluttered

Too many files and folders present on your desktop cause the slowing down your machine. You must arrange these files orderly into folders and should keep them away from your home directory. You can access to them by creating shortcuts to them.

2-Having Corrupt Preference Files

Corrupted preference file makes the program’s working weird or slow. If your system slows down while using any particular application, then delete its preference file and re-load the application again. Applications can create a new preference file if it doesn’t exist.

3-Smart Playlists set on iTunes

Smart play lists can usually turns the iTunes slower since they have to be reloaded each time you launch the application. You should disable live updating to solve this. For this go to the File menu, now click Edit Smart Playlist and remove the tick mark from Live Updating.

4- Lots of Widgets on Dashboard

Each Widget on your Dashboard occupies some memory of your machine. Memory usage by your widgets can be checked using the Activity Monitor. You should remove all the unused and memory eating widgets by going to the dashboard control panel..

5-Caches, Log files and Temporary Items

You should frequently clean up the Log files, Caches and Temporary Items. You can use use any application like OnyX to do this task. You can also delete the files manually.

6- Wrong Firmware creating problems

Wrong Firmware can also create all sorts of troubles; so you should keep your software up-to-date on Mac. You can do this by Going to Apple menu, and clicking Software Update. Softwarre update scheduling can also be done. This can be done by going to System Preferences > Software Update and tick the Check for updates option. You can also set or adjust the frequency of the checks by selecting appropriate options in the drop down menu.

7-Insufficient RAM

By upgrading your RAM you can increase the speed of your system. Clicking on the System Memory tab will show a pie chart. If the chart is showing mainly red or orange color then your system is running out of RAM. The increasingly growing Page Ins and Outs numbers also indicates that it is time to upgrade the RAM.

8-Permissions Conflicts

Many problems like slow loading of application can be solved by repairing permissions. You can fix disk permissions by using the Disk Utility application. For this select the start up disk then click the First Aid tab and now click on the Repair Disk Permissions tool.

9-Hard Disk With no extra space

Mac uses hard disc free space as Virtual Memory to free up the RAM. So, you should make sure that at least 10% free space should be present in your hard drive for this purpose.

10- Unnecessary Login Items

Your login items should not contain any unwanted or less used programs. The changes in your login items can be made by going into System Preferences, then selecting the Accounts tab and selecting the Login Items.

11- Unused System Preference Panes

You should clear all the unused preference panes for freeing up the memory and disk space. For this select Other in the System Preferences for removing the unnecessary ones. You can either remove inactive it from its menu or can fully delete it from the location …/ Library / Preference Panes.

12- Unused Applications functioning

Each running application consumes your memory and CPU resources.So close the unnecessary applications which you are not using at the moment. You should manage the memory leaking applications in the Activity Monitor.

13- Animated or slide show Wallpapers

Animated or slide show wallpaper will affect your machine performance. So, better turn it off.

14- Firefox Overloaded With Add-ons

Many Firefox extensions can also add the burden. You should uninstall any Add-ons that you no longer want.

15- Improper Internet Settings

If your browser works slowly then clear its cache and delete all your history. If you are using Safari then go to its main menu and click the option Reset Safari, also check the Clear History option and Empty the Cache option and finally reset it. If you have Mozilla Firefox then you can do this by selecting: Preferences > Privacy > Clear Now button.

16- Favicons Saved in Safari or Firefox

You can improve your browsers performance by deleting all cached Favicons saved.

In Safari you can find these files in ~~/Library/Safari/Icons.

17- Bulky Mailboxes

A massive mailbox which contains thousands of messages takes a long time to load. So, delete all the unwanted messages and break larger your larger mail boxes into smaller folder.

Before doing all these operation make sure that you have taken the backup of all your important data.