I think you must be doing something very wrong if you measure less than 1 Mpx/s for fjxl. It should reach at least 50 Mpx/s or so, and of course more if you are using multiple threads.

Anyway, yes, we should improve decode speed — for example, currently every sample value is unnecessarily converted from int to float and then back to int, and obviously that causes some avoidable slowdowns.

I agree that MT PNG does have a gentler transition path than introducing a new format. The point remains though that you need to somehow get both the encoder side and the decoder side upgraded to benefit from the advantage, which can be hard given how many existing deployments of png there are. Any system that has to take arbitrary png as input cannot rely on MT decode being an option, etc.

Also one disadvantage of MT PNG compared to JXL is that it splits the image in stripes, not tiles. For full-image decode, that doesn't matter much, but for region-of-interest (cropped) decode, tiles are a bit more efficient (no need to unnecessarily decode the areas to the left and right of the region of interest).