Moved from the Creative Cloud to the Photoshop forum. They will be able to help you here.
Since you are on a Mac, you can't directly change your display driver version, but you CAN double check to ensure you're on the latest OS update.
Some things that might help you regain performance:
1. You can go into the Photoshop - Preferences - Performance dialog, choose [Advanced Settings], and see whether any of the alternate drawing modes (Basic/Normal/Advanced) changes Photoshop's behavior. Remember, you need to quit Photoshop and restart it to properly test changes to these settings. I'd also suggest checking to ensure that Cache Levels is 2 or higher in that Performance panel.
2. Reset Photoshop preferences by pressing and holding the Command - Shift - Option key combination immediately upon cold-starting Photoshop. If you're quick enough (and you have to be very quick to get the keys down), you'll be prompted to confirm deletion of the current preferences. This will lead to Photoshop regenerating an all-defaults preference set. You can still restore a named workspace, but you'll have to re-do all your custom preferences.
>> the CPU use is near %100 even when no task is being processed.
That either means that you're paging heavily due to low RAM, or you may have a corrupt font or OS font cache.
First, try resetting Photoshop's preferences.
If that doesn't work, then you'll need to troubleshoot the fonts and OS font caches on your system: http://helpx.adobe.com/photoshop/kb/troubleshoot-fonts-photoshop-cs5.html