I'm using Photoshop CS6 for some time now. I had enabled GPU usage earlier and I remember that Photoshop ran absolutely smoothly without and lag what-so-ever. This morning, all of a sudden, it has become extermely laggy. I don't understand the reason behind this. This had happened to me before as well but it became alright after a few days. This also happened to me once while using Photoshop CS5. I didn't install any plugin and didn't change any software. I also want to point out that I had 13.4 driver version when Photoshop was working fine, and now too I have the same driver version.