THREE.EquirectangularReflectionMapping tells the renderer that an environment map is in the equirectangular format. Since only the cube map format for non-PBRs and the cubeUV format for PBR materials is supported in shaders, equirectangular textures have to be converted be rendering. This is done automatically by the renderer.
Looking at the code, WebGLRenderer.initTexture() treats equirectangular textures as normal textures so they are not converted to a different format until you actually render with it. However, the initial upload of the equirectangular textures during the conversion should be eliminated.