source: polyhaven.com/hdris
When three.js loads a .hdr file, it first processes it for efficient rendering – it creates a mipchain of smaller resolutions and converts the 32-bit float data into 8-bit rgb and a scale factor in the alpha channel: rgbd explained.
The resulting data is much smaller than the original hdr file. For faster load times I extract this data into pngs and load those instead, this saves load-time and processing-time without quality loss
You can see how to use these pngs here: EnvironmentManager.hx