There are some web apps out there that are sluggish as hell if one doesn't have the latest beefy machine. Gmail or Google Doc running on old hardwere are examples of this. "Mobile web" apps are very sensitive to DOM manipulation performance too.

I'm not saying that all sluggish performance in web apps is caused by inefficient DOM manipulations, but having a DRY and efficient way to express DOM manipulations could help a lot in that regard =D