There's some pretty sluggish websites out there due to poor use of js libs. Browse them on a phone or netbook and it all adds up.
You'd certainly manipulate hundreds of DOM elements at a time, consider say a twitter stream, where each post has "10 seconds ago" marker, and they all need updating.
The bottleneck in that situation would be finding the elements, not updating them. If there is any worry about performance, the change would be to cache that list of elements rather than searching again and again, regardless of the library used. (Unless your library was very clever, and could cache the results for you. I'm not sure what browser support there is for ondomupdated events, which you would need to watch for this to work in a general fashion.)
You'd certainly manipulate hundreds of DOM elements at a time, consider say a twitter stream, where each post has "10 seconds ago" marker, and they all need updating.