PSNR is not a very good test of human perception though, and optimizing for PSNR has had unfortunate consequences on image resizing algorithms IMO.

Whether downscaling a high res image is an excessive load on the end user depends a lot on the end user. From what I understand (but I’m not an expert, so please correct if this is wrong), bandwidth is the main bottleneck not only on I/O latency but also on CPU use, not image rescaling. I’m guessing even mobile phones of the last few generations don’t even break a sweat when downscaling images (can’t they use GPU for this?). As always, it would be a good idea to actually test CPU use / latency / battery drain from rendering images at different sizes/JPEG quality levels on the target client device.