Image crop and preview for most files are significantly faster in this update. Previously all files were uploaded to the server and appropriately resized versions were downloaded from CDN for preview. This meant that the user was forced to wait during file uploading, image resizing and delivery over the network.
Starting from version 2.6.0 the widget attempts to render local files as images using HTML5 APIs. This gives the ability to deliver, crop and preview immediately. Of course, browsers are not able to open all image formats which our servers are. In the case files can’t be rendered on the client side, they will be handled in the old way.
This significantly speeds things up when possible (vastly improving UX) but will not affect other cases.
You can find full changelog entry here