The main problem is your computer lack power to work with a rotation and green screen plus probably a background picture or a video acting as background.. When your work with Magix use half or quart resolution in the program also think to close others programs that you don't need.
Try first exporting the upside down rotated upright video to Magix mxv format. Then import that into a new project. That way the program won't have to work hard on the rotation.
Note that this method applies to anywhere that you have heavy effects that slow down processing. You just have to keep track of the source project(s) in case you want to go back to edit the original.