In order to support incremental loading (introduced by v1.5.0), you just have to go to this page in order to get a tool to convert your .babylon file to a .incremental.babylon file alongside associated resources (.babylonmeshdata and .babylongeometrydata files).
These files can be used just like a standard .babylon scene except that they will allow Babylon.js to load meshes and textures on the fly. This means that the meshes and the textures will not be loaded at startup but only when they become active (when the active camera can see them).
You have to put the .babylonmeshdata and .babylongeometrydata files in the same folder as the .incremental.babylon file.
You can find a demo of an incremental scene here: https://www.babylonjs-playground.com/#JA1ND3#84
This script can be used to generate incremental files from babylon scenes. It can be ran directly from node or it can be integrated into build scripts. More information about how to install and configure the script can be found here.