This should not take a long time. Normally it happens virtually instantly for me. For some images, like a 63 MB linescan image I sometimes work with, it can take 2-4 seconds to read it in and downsize it for display. Perhaps it's causing a delay because you picked a reserved function name "image" for the name of your image variable. When you do that it destroys the image() function and maybe that causes some delay. Or it has to figure out what to do. Pick another name so there is no conflict, like baseFileName or something.
baseFileName = uigetfile([folder '\' ext]);
old_image = fullfile(folder, baseFileName);
