Transparent Scrollable Overlay Help

Hey there, I am working on a GUI through GUIDE and instead of imfuse()'ing the images together, I want to get an overlay so I can adjust the transparency. The imgStackCopy is the original image stack and the savedImageStack is the binary mask stack. I have the following code, but it is very slow and the mask does not disappear while scrolling so the updated one will show for the next slice.

            newImageCData = handles.imgStackCopy(:,:,handles.currentSliceNum + scrollUpDown/3);
            handles.imageAxes.CData = newImageCData;
            handles.currentSliceNum = handles.currentSliceNum + scrollUpDown/3;
            set(handles.sliceNumberText, 'String', handles.currentSliceNum);
            hold on;
            a = imshow(handles.savedImagesStack(:,:,handles.currentSliceNum + scrollUpDown/3));
            set(a, 'AlphaData', 0.5);

Is there any way of better overlaying the images together so the transparency can be adjusted and it is fast?