Dropbox, box, or any other cloud service will be as slow as your general internet connection usually. If you are transferring massive files like multi-gig videos, raw images, etc locally, then your better bet is to simply sync them over your LAN. There are lots of easy ways to do local file sharing depending if you are a PC or Mac user - it’s built in to both OS’s
If you are not on the same network/same location then that isn’t an option.