Several factors can make your Mac slow and there are quite a few things which you can do to try and remedy the problem. One is to repair disk permissions, which is easiest and i recommend you to try this first if you haven't did this before. First go to your launchpad or applications folder, under the 'other' tab open 'disk utility' and select your hard disk by the left column (by default it is named as 'Macintosh HD'), under the 'First Aid' tab you will see a button called 'Verify Disk Permissions'. Click that and let the computer do it's job. After it has finished (it will be stated in the text bar above this button) then click on the 'Repair Disk Permissions' and wait for it as well. When it's done you can go over to the right side where you will see a button called 'Verify Disk' and click on that as well. Let it finish and repeat this on your other hard drive selection on the left of the column (where you selected the 'Macintosh HD') This time you should only be able to click on the 'Verify Disk' and 'Repair Disk' button, click on 'Verify Disk' first then click 'Repair Disk' when the verification is done. When all this is done, close the disk utility and see if your computer is performing as it should. If it's still not working then it might be a more severe issue on your computer and you might need to reformat it. Get back to me after doing this and we'll see if you need to do something else.
How large is your HD & how much space do you have left on it?
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Sluggish Finder - Bunch of icons on your desktop? An overcrowded desktop slows down your Mac.
Get rid of needless Finder calculations - Open a window in Finder/View/Show View Options: ensure "Calculate All Sizes" option is unchecked.
Internet related - It's likely that your broadband connection is the bottleneck. You can check your speed on http://www.speedtest.net to see how fast your connection is. If your web browser specifically performs slowly, quit and relaunch it.
If browser remains sluggish, empty its cache.
Firefox/Preferences - select the Network tab of the Advanced preferences and click the "Clear Now" button in the "Offline Storage" area.
Application related - Launch "Activity Monitor" - Applications/Utilities - click the CPU heading and see what float to the top. If an application takes up a large chunk of CPU and won't let it go, it could be dragging down your Mac's performance. Quit it by clicking the Quit Process button at the top of the Activity Monitor window.
Too little ram - max out your ram.