If Photoshop is to slow for you, then there are a number of things you can do. Before you do any changes to settings, please make sure that Photoshop has enough RAM.  As soon as Photoshop starts paging to your hard drive for memory, everything slows down dramatically.

Optimize Photoshop Preferences

• Performance / Let Photoshop use 70-80% of the MB.
• Performance / Cache levels.  Try using a smaller value here, do try as low as 1.
• Performance / History states.  Try a smaller number if needed (as low as 20).
• Performance / Use graphics processor.   I leave this checked for faster performance with certain features (liquify filter, adaptive wide angle, etc). 
• Scratch Disks.  If you have multiple drives, you can use them as virtual memory.  This can be especially helpful if your primary drive is full.  This also helps to add a drive that is different from the one on which Photoshop is installed.

Free up resources for Photoshop

• If you are on Windows, run the 64-bit version of Photoshop.  The 32-bit version is limited to 3GB of RAM, which just doesn’t cut it.
• Close other photos other than the one you are editing in Photoshop.  This frees up more RAM.
• Quit other programs that you don’t need running. This frees up more RAM.
• Clean the junk on your computer with a program like CCleaner or clean manually (free up space on the hard drive by emptying trash, defrag your hard drive, etc).   This can speed up virtual RAM.
• Keep in mind that background programs (such as anti-virus) also impact system performance.  On windows, click on the Start menu and enter “msconfig” to see and disable background programs

Invest in new hardware

• Install more RAM.  I personally run with 16GB. There is a significant boost for most users between 4 and 16GB, after that the benefits fall dramatically – unless you are working with very large images.  To check if RAM will benefit your machine, click the arrow below your image that normally shows the file size and choose “efficiency”.  If you have taken the steps above and still see efficiency is less than 100%, you are using scratch disks and will benefit from extra RAM.
• Invest in a Solid State Drive (SSD).  These are dramatically faster than spinning drives, and the price points have come down significantly.  Even if you have more than enough RAM for editing, this helps launch Photoshop more quickly.
• Photoshop doesn’t currently benefit much from multiple cores; go for a machine with the fastest GHz processor.  A higher speed processor with fewer cores will run Photoshop more quickly than multiple cores at a slower speed.