I figure that doing less work generally requires less power, but some developers are crazy and act like they own the whole CPU, generate tons of heat, stress out the cooling system etc. with multithreaded bullshit when they did NOT need to be multithreaded.
Some multithreaded workloads will use the full logical processor count to pin every core and that can really suck on low-end systems. I think aside from balancing multithreading overhead, this is one of the reasons why some more advanced software (such as 7-Zip) lets you select the number of threads to use for a workload, even if you already know and accept that it'll be expensive until it's completed.
Lots of multithreading things I come across as a developer have a default of something like 4 threads, or only use a certain fraction of logical cores up to a certain maximum, etc. because it's generally more polite not to always pin every core.
Nowadays the focus seems to be on video game-style interfaces that redraw every frame. The justification for this is typically that computers are so fast they can now come up with the entire interface every frame, so why not take advantage of that and forget all this retained-mode complexity?
They don't realize that this is a classic case of the developer's machine fallacy, where the developer has a machine so vastly more powerful than any of their users' that they produce a terrible experience for said users. This is also why games these days typically require upscaling even on 5090, because developers usually have that or something even more powerful and they think the bleeding edge is pushing even THAT machine to its knees.
Developers should be testing their software on the most entry-level, bottom of the barrel Chromebooks from 2015, running the heaviest and most bloated operating system they can think of. If people still optimized their software for that, then maybe computers would still feel almost as fast as the Apple II from 1977.
https://danluu.com/input-lag/