I do screen grabs of woodworking projects I wish to make and save them to Lightroom CC. I selected one of these screen shots while viewing in LR CC and selected to edit in Photoshop. The pic opened there. I wanted to remove the background and place it on a graph paper background so I could better estimate dimensions. I used the quick select brush to select areas that I deleted. I enlarged photo and tried to erase more of the background. My eraser did not work and as I moved the picture around the screen using my mouse, i found that the picture was moving in a very jumpy fashion and often, it would move on its own or continue to move after I stopped using the mouse. My iMac processor is 3GHz Intel Core i5, with 16 GB memory and a Radeon Pro 555 @ 2048 MB. Is my iMac just too underpowered for PS?