I found this on another webpage. Maybe this will help until some of the guru's on here can post.
The first reason for your Macbook Pro running slow is the processor. When there are too many applications in the background its sure that your laptop will have a slower performance. You can find out which are the applications that you currently use easily. Click on the Applications icon, then go to Utilities and then select Activities Monitor. Now, you need to simply close all those applications that you havent used and your laptop will run faster.
The next reason for the low speed of your laptop is the disk permission. To fix it you need to follow some steps. You need to select Applications and then select the Utilities option. There, you will select Disk Utility where you will have to click on the disk drive that you want to repair. Then click on the Repair Disk Permissions button.
The preferences of the energy saver may cause a speed decrease. To solve this issue you need to select Applications section. Then you need to go to System Preferences and access Energy Saver section. Check if your laptop is set to Better Performance and not to the Better Energy Savings option. You should know that the new version of the Mac OSX of the Macbook Pro doesnt have this setting.
If you have the Web Sharing on, this may be a cause for your Macbook Pro running slow. To change this setting you need to access Applications and to go to System Preferences. There you need to select Sharing. If the Web Sharing option is selected, you need to uncheck it.
Another reason is the incompatibility between your laptops software and the processor or between the software and the graphics card. You need to see what type of processor you have. In order to do that, you need to access the Apple Menu and see About This Mac. To view the type of the graphics cards, you need to access again Apple Menu, then About This Mac and click on the More Info button. After that, choose Graphics/Displays. Search in the software documentation the compatibilities. Then you need to quit those applications which are not compatible.
Another factor that influences your speed is the RAM (memory). This may decrease the speed because there is not enough memory for the running applications. To resolve this problem, you need to put more RAM in your laptop or replace the software.
You can find out the causes yourself, because it is very easy to do this. The causes of your Macbook Pro running slow might be resolved quickly.
Link to original article.