Well, I agree that there are specific program architectures that can cause problems, but I haven't had the experience where "toss and rebuild" is the bottleneck

Well, I agree that there are specific program architectures that can cause problems, but I haven't had the experience where "toss and rebuild" is the bottleneck. One of the other responses nailed what is a more common problem IME -- network activity and UI activity on the same single-threaded event loop. Firefox and Chrome both have this problem (or what amounts to it essentially). For example, in Chrome: http://code.google.com/p/chromium/issues/detail?id=113389 http://code.google.com/p/chromium/issues/detail?id=113389 This bug has gone unfixed for YEARS (older ones were marked dupes of later ones). The way we write software now is just too difficult to get right. Once you have 1M lines of C++ code there are going to be crazy performance bugs that nobody can fix. Chrome has some of the best engineers on the planet and they're not immune to this by any means. I guess I would amend my original answer to say that "software size" is the problem. Everybody thinks they know how to write performant software. But that's only when you can fit everything in your head. I would say that "toss and rebuild" isn't the #1 culprit because the web browser is the ultimate "toss and rebuild" architecture -- that is, every request and response stand alone. Yes web pages can be ungodly slow and unresponsive. But it's possible to make them responsive if you really simplify -- i.e. Google or craigslist. In theory we could have a stateful web and make it faster. But I think our collective programming skill and our languages/tools just aren't up to the task. I think the web exists as it is, and is as popular as it is, because lots of people with "domain knowledge" could write dirty PHP scripts and such. They aren't fast but they get the job that users want done.