the classic slide editor lets you easily select a layout and then select what is going into each section and it automatically sizes images, gifs, etc to the section you've selected. This makes it SO much faster to utilize images.
On the new slide builder you have to take many more steps to add an image then it goes into a random spot on the slide and when you move it, you end up distorting the image. Ideally, the image should populate into the section of the slide you've selected and THEN you can edit it to move into another location. The default location for an image to populate on the new slide editor is not intuitive so its creating extra steps for users. I made a screen recording of this to show the difference in time: https://watch.screencastify.com/v/xUbR3YN37RwuUvmwmHww
The personalization in the new slide editor is amazing but the overall UX design is not user friendly or intuitive. This feature needed more beta testing and improvement before its roll out.