I'm always impressed by how well hugely popular games like League and CSGO run on bottom-tier machines. Maybe it really isn't all that impressive and its just the number of poorly optimized bloatfests out there that skews my perception
As with all optimization it is hard: as you try to squeeze more out of the machine your code gets less and less elegant.
Still, in my experience any codebase that pushes a large amount of technical boundaries tends to be full of huge "WTF is this" moments that require explanation. It is an achievement if you can create a decently powerful codebase that can survive rapid changes and additions. It's much more common, but I've seen and worked on AAA codebases that would break if you blew on them.