I hear what you are saying. I have a very large number of cores in my workstation and it's disappointing/frustrating how many applications—most poignantly, Mozilla Thunderbird—are in practice single-threaded. Even using Firefox with Electrolysis and a bumped dom.ipc.processCount setting yields an experience that seems routinely stuck waiting for a single user interface thread to do something.

And multithreaded JavaScript for webapps seems like a pipedream. Virtually no one uses Web Workers because they were designed to avoid sharing data between threads, making them inordinately cumbersome to use in any routine/real-world cases.

There is a lot of room for improvement across many fronts. But I am hopeful it will occur sooner or later because I suspect adding more cores will be easier than increasing the speed or each core for the foreseeable future.