Hmm... my old x86 work machine (C2D, 2 GB RAM) capitulated much, much sooner because of insufficient memory than CPU load due to browsing. And before you ask, the number of open tabs was not that relevant in my use case, but what types of website I browsed.
Here's what I would have in mind: Run the following setup, as an example of ultramobile laptop replacement, on an external monitor:
First, start your capable browser to:
1) work two very image-heavy Twitter feeds,
2) maybe a third tab for working with Google street maps (street view!), and
3) another tab or two for communication (e.g. e-mail, forums, social media).
All the while, 4) having a knowledgebase/note taking application à la Org-mode (stand-alone) or TiddlyWiki (in another browser tab, running on node.js) on stand-by, while you 5) periodically keep a couple of pdf and epub files of typical sizes open for a while as well, e.g. files in the range from 5 MB to 200 MB.
Run the above setup for at least half an hour to an hour and do (simulate) some real work with it. Phase out nos. 1, 2 and 5 if necessary as they can be stress tests on their own.