Multiple layers of complex abstractions and sloppy coding account for a lot of it. We write code as if it had no hardware constraints, so we get megabytes-size web pages running a slow interpreted language in a browser built on multiple layers of frameworks and libraries. Over the last several decades inefficient and bulky programming has consumed all of the gains in hardware, to the point that modern applications feel much slower than similar apps from the '80s.

Knuth did not have that in mind when he wrote about premature optimization.