the fastest way to clean a horse quickly- be it when running late for a show or trying to hose off a mud caked
horse while your friends wait for you to tack up- is with a spray & scrape method. Instead of bathing the whole horse and going back over trying to rinse dirt out, you should work section by section. You'll need a hose with decent water pressure and a one handed sweat scraper. Hot water works best but cold will do if it's warm enough outside. Hold the hose and spray against the grain of the hair, then squeegee down, hose the same place, squeegee, hose, squeegee, etc till water squeegees clear. Start at the neck and work your way down and back. This works similar to washing a dirty towel- wringing it out between washes cleans more effectively than just rinsing continuously. The sweat scraper in this method works like a wringer in that it scrapes a cascade of water off the horse, taking with it dirt and mud.
You'll get soaked and filthy in the process of using this method to get your horse clean quickly but it works good and works quickly , even on a winter coat!