A number of different issues -- ranging from too many open applications to a sluggish Internet to a full hard drive -- can cause your iMac to slow down. Perform a few troubleshooting procedures to speed up the computer. If necessary, upgrade the iMac's hardware to make it spritely again.
Before proceeding to complicated troubleshooting, perform some basic tasks to improve the speed of your iMac. Restart your computer at least every few days. Close any unnecessary applications. If you have many background tasks running -- such as downloads and large print jobs -- quit any that are unnecessary. The more apps and processes that are running at once, the harder your iMac has to work because it has to spread its memory and processing power around among multiple tasks.
Use Activity Monitor to Check Usage and Quit Processes
The Activity Monitor program that comes with your iMac helps you keep tabs on what's running on your computer and measure how much strain it's putting on your iMac's resources. Activity Monitor is located in the Utilities section of your iMac's Applications folder. Click the "CPU," "Memory" or "Disk" tabs in Activity Monitor to view how much of your system's resources a process is using. Higher numbers indicate an item that's using more resources. The higher the number, the more likely that process is slowing your iMac down. To quit something, click it on the list of processes and then click the "Stop" button in the Activity Monitor. If you're unsure about how to use Activity Monitor, click "Help" and then select "Activity Monitor Help."
Troubleshoot Slow Internet Speeds
If you're finding Internet-based applications slow, it may be your Internet connection rather than your iMac itself that's slowing things down. Downloading and streaming multiple items at once can dramatically decrease the speed of your Internet. If your iMac is located far away from your wireless router or modem, move it closer to improve the speed of your connection. Restart your router or modem to re-establish its connection to the Internet.
Hard Drive Issues
Your computer's hard drive slows down as it gets closer to filling up. To see how much space is left on your drive, click the "Apple" menu in the top corner of your iMac's screen, select "About This Mac," click the "System Report" button and then select the "Storage" tab. If you're running low on storage space, back up nonessential files with Time Machine, or use the Finder app to move music, movies and other personal files onto an external hard drive.
Significant drops in your iMac's speed despite ample room on your hard drive may indicate that the drive is failing. If the computer is still under warranty, take it to an Apple Store to have the drive replaced.
Upgrade Your Hardware
Upgrading your iMac's memory, or RAM, and hard drive is the most effective way to speed up the computer. However, it's also the most complicated and most expensive. Because of the design of modern iMacs, it's difficult for users to upgrade the hardware themselves. If other troubleshooting solutions don't speed up the computer, take it to an Apple Store or a licensed Mac repair shop to have its hardware upgraded.