|Were you previously using Photoshop without the slowdown? |
If so, you need to clean your system of junk and also probably shut down some unnecessary programs from running at start up.
If not, then adding RAM "may" help with the problem or it could be related to your graphics subsystem.
I don't know much about photoshop. Perhaps this thread needs to be moved to another forum. If so, that can be done if requested.