That's probably because photos are an incredibly common task on phones and so the software has gotten a huge amount of optimization attention. I don't think it's a hardware issue.
Phone UIs in general get more love just because with that tiny screen and given the normal causal use cases users are far less tolerant of slow or poorly designed UIs on phones. The squeaky wheel gets the oil, and on mobile any UI issue is instant death. On desktop it's just an annoyance.
The point was to give an example that desktop/laptop computers do not always result in better performance than phones.
The GP didn't seem to believe it was possible that a phone could be "less laggy".
In line with your own remarks, there are a number of common use cases that have been optimised on phones in a way that they never were on desktop, resulting in a superior user experience on the phone vs the desktop.
Photos on my iPhone SE (2016) or 9.7" iPad Pro (2016) is significantly more performant than Photos on either of my MBPs (13" & 15", 2015).
Like massively so, to the point that I absolutely loathe having to use my Mac whenever I need to do anything with a photo or video.