Open Applications

Perhaps the easiest problem to fix with an iMac running slowly is to use fewer application at once. RAM (Random Access Memory) is used by the iMac to store data in "running" programs; the more programs running at once, the slower the iMac seems. If you have many running programs, quit some of them and see if performance improves.

File Problems

Another common problem is an abundance of small errors or issues in your iMac's file system. In your Applications folder, locate the Utilities folder, open it and launch "Disk Utility." Run "Check Permissions" to fix issues with file permissions. Once that task has been completed, you may find your iMac runs more quickly.

Disk Problems

Disk Utility can also be used to check for and repair other problems with your hard disk and files; many of those problems can only be repaired if you run Disk Utility after restarting your iMac from its installation CD or DVD. Third party tools like Micromat Techtool Pro can check for additional or more complex problems disk problems and can optimize the files stored on your disk.

Delete Files

If your iMac's hard disk is nearly full, that can cause slowdowns, as the Mac OS needs free space on the hard disk for a variety of temporary files. Back up your files and then delete archived documents, music or videos so that at least 10 percent of your hard disk is free space.

Login Items

Sometimes an iMac can be slowed down by "login item" applications and utilities that are launched when you start up your Mac. To see your login items, choose the "Apple" menu, and then "System Preferences." In System Preferences, choose "Accounts," click your user icon, then click "Login Items." You can select an item and click the "-" button to stop it from launching when you log into your Mac.

More RAM

If software solutions don't help, your iMac may benefit from more RAM. Adding RAM can increase your iMac's ability to run programs at once and speed it up overall. Most iMac models (particularly iMac G4 and later) offer an user-serviceable method for upgrading RAM; visit Apple's Support site for details.

Source:

Apple Support