[BuzzwordBingo]

Looks like a for loop iterating over each pixel one row at a time.

What makes this fast?

[viraptor]

It's fastest because others are slower. It doesn't necessarily mean it's fast :)

[time0ut]

It does at least parallelize the work across available cores.

[arendtio]

This is the line that makes the difference:

https://github.com/n7olkachev/imgdiff/blob/master/pkg/imgdif...

[srgpqt]

Dunning-Kruger syndrome, probably.