Hmm..I know that it's time consuming to watermark using Windows Movie Makers. After adding your watermark, don't save it as HD display, instead save it by clicking on "Recommended For This Project". It will be saved, quickly.
Actually, if you place your cursor on all 3 project saving options, it will show you, "Bit Rate" And "Estimated File Size". Opt for minimal paratmeters, Yes quality will be reduced, but it is still good enough.