It has been at least a few months since I did some testing on my MacBook Pro m4, where I converted a bunch of camera jpegs to jxl losslessly, and yeah the jxl files were smaller, but the decoding/viewing experience on macOS was worse than with jpegs. Thumbnails were missing, and just browsing through a folder of them with QuickLook was unbearably slow compared to jpeg, perhaps at least few hundred milliseconds or even longer than a second, when the JPEGs were instant. Idk if that’s just Apples implementation not good yet, but that’s why I at the time abandoned converting all my JPEGs to lossless jxl