Personally, I find that the 1080p -> 4k upscaling works very well on this TV. I haven't tested lower resolutions since.

The Canadian site RTINGS gives a score of 8.0 for upscaling:
https://www.rtings.com/tv...

I wouldn't expect too much when upscaling SD material (720 pixel lines) to 4K (3840 pixel lines). The best TV cannot calculate a high resolution from a low resolution. If the difference is also not an integer (3840/720=5.33), it cannot simply be multiplied, but interpolated and then more or less interpolation errors are added. To cut a long story short: Watching SD material on a 4K TV is more or less difficult, in any case you have to keep the SD viewing distance, i.e. about 5-6 metres.