How to troubleshoot slow web browsing in Ubuntu VBox guest (Windows host)?

How to troubleshoot slow web browsing in Ubuntu VBox guest (Windows host)?

I have an Ubuntu 12.04 LTS VirtualBox guest running on a Windows 7 host.
Until recently it had been working flawlessly1.
Since a few days ago, however, web browsing in this Ubuntu VBox has been
extremely slow (so much so that many pages never finish loading). This
sluggishness is confined to the Ubuntu VBox; I experience no slowdown at
all in the Windows host. (This rules out server-side problems and problems
with the internet connection.)
I have reason to believe that the problem has to do with the Ubuntu VBox's
DNS resolution, because the sluggishness started right after I had to run
sudo dpkg-reconfigure resolvconf. I posted a question about this at that
time.
That said, I can't figure out what I could have broken by doing this,
since, besides answering 'Yes' in the dpkg-reconfigure resolvconf dialog
(described in that earlier question), I did not make any other changes to
my DNS resolution configuration.
In any case, it was after doing this sudo dpkg-reconfigure resolvconf and
rebooting the Ubuntu VBox, that I noticed that web browsing had become, on
average, very slow.
A typical experience is that, using Firefox (for example) I do a Google
search (which is usually reasonably quick2), then I click on any one of
the top hits, and then the looong wait begins. This wait is long enough
that, before it's over, I have time to switch to the Windows 7 host,
repeat the same Google search (using Firefox again), click on the same hit
that I'd just clicked while in the Ubuntu guest, instantly retrieve the
desired, read whatever I needed to read, and switch back to the Ubuntu
guest. (If nothing else, this confirms that whatever the problem is, it is
confined to the current configuration of the Ubuntu guest.)
How can I troubleshoot this problem further (and hopefully fix it)?



1 As an aside, let me express my admiration for VirtualBox. Wow, what an
impressive piece of software!
2 Interestingly, the first few Google searches I did after reconfiguring
resolvconf and rebooting were slow-as-molasses, but eventually they got
faster. My guess is that (consistent with my suspicions of the current DNS
setup) this observed speed-up is due to caching of some key IP addresses.