Quote Originally Posted by tpruzina View Post
... (human eye detects changes in color shade much easier than one bright pixel in dark picture)...
It's actually the other way around. The eye is much more sensible to changes in brightness than in color. This is the reason why the chroma components are often subsampled (to reduce bandwidth without losing much detail).

Quote Originally Posted by CrystalGamma View Post
Because AFAIK it is often done by the GPU (depends on the architecture of course).
I never saw an implementation that did decode on the JPEG on the GPU. Not saying it's impossible, but considering the advantages vs disadvantages I doubt anyone would implement that other than for educational purposes.

JPEG encoding/decoding is faster than PNG because the algorithm is just less expensive in computational resources.

Quote Originally Posted by CrystalGamma View Post
Also the bigger size of PNG naturally leads to longer decoding time ...
Certainly plays a role, however it's irrelevant.

Quote Originally Posted by CrystalGamma View Post
Why are we even discussing this? These two format have completely different purposes.
Because someone asked why is JPEG more prominently used over PNG and why not use PNG for everything