Anyone knows what they do different to achieve such speed?

I thought "I doubt it is faster than FastStone" but when I went looking for benchmarks all I could find was this forum post from 2011.

https://www.donationcoder.com/forum/index.php?topic=25334.0

Which does confirm that Faststone is faster but much water has passed under the bridge since 2011.

That's an interesting benchmark, but I am more interested in a cold start test on a much smaller file (think 2MP, not 1200MP), because that's my usual use case.

I agree. That is another problem with the benchmark- it gives the gold medal to ACDsee, which in my experience has the longest cold start of the three.

The batch editing is also handy

I was a fan of AcdSee, currently using DirectoryOpus, never got close yo the feel as in tbose days... so I get it...