Yes, it does. I wrote a small HTML5 offline-capable web app, processing a ~100KB JSON file with around 200 items. For ease of development, I initially used a J

Yes, it does. I wrote a small HTML5 offline-capable web app, processing a ~100KB JSON file with around 200 items. For ease of development, I initially used a JSON query library, which was perfectly fast on my laptop, a bit pokey on my 3GS, and - as I found out rather late - amazingly slow on an iPhone 3G. I removed the library in question and rewrote my queries to use JS1.6's filter function; along with some template optimization, I managed to get the delay down to somewhat reasonable... but it's still a touch pokey on original 3G iPhones and older Android phones for my tastes. There's likely more optimization I could have done, but I tested far too late on slower devices and just didn't have enough time. In the future we're looking at either writing a native app or using HTML5 database storage - but most importantly, testing far more regularly on slower devices. We've also reconsidered writing some truly large applications in HTML5, as we're rather worried devices will just be too slow, and we won't find out until late in development. (The app in question is http://pax.expojunkie.com/ http://pax.expojunkie.com/, if anyone wants to take a look.) tl;dr: Yes, JavaScript performance can be a problem. If you're writing a JavaScript-heavy site, get the slowest device you expect your users to have and test it regularly.