Web apps can't hope to achieve the kind of responsiveness BeOS offered, at least not for a few more years. Unless you have near-zero network latency and multithreaded JavaScript apps, you cannot possibly build responsive web apps. We will get there, sure, but it will take a while.

BTW, I know this because I use Haiku on my PC. Haiku is responsive because it has been built to be that way from the ground up :)