Our friend, Kaliya, has made a preload plugin for RPG Maker MV! 
This is a RPG Maker MV plugin that is designed to add a preloader and caching system to your game. Are you tired of having your sound not play right away? Are you tired of some images not appearing when they should, because they’re still loading? Well this plugin is for you! It is designed to allow the developer to selectively preload certain things globally, or when the player loads a map, or on demand via a few scripts calls! The plugin also has a basic caching system to help alleviate the issue of MV using too much memory if used properly.
This plugin was designed and developed for RPG Maker MV >=1.6.1. It will not work in versions lower then this.