Slow-mo video is not slow motion after uploading to Google Drive
Slow-mo videos are videos recorded in a higher framerate. To make them slower, Photos plays them back at a lower framerate. This is nothing to do with the file and everything to do with the playback of the file. When you upload the file elsewhere, it's still in a higher framerate, but you're playing it back at this higher framerate. If you want to watch it slower, use Photos to adjust the playback speed.