Low, medium, or even high — it depends a lot on the options selected and the size of your files. Before uploading, optimize your video poster with TinyPNG, and compress your mp4 file with a tool like VideoSmaller. Autoplaying videos may have a higher impact. If you use an external embed option, it depends a lot on the speed of that service. Thumbnail is usually the faster option if you optimize your image because no other files will load until the user click to open the lightbox.