hand-wave-y cache performance data

We've had lots of talk about how our cache might be slower than other
browsers, but not much data.

Here's some results from the latest Tom's hardware "Web Browser Grand
Prix":

  http://www.tomshardware.com/reviews/macbook-air-chrome-16-firefox-9-benchmark,3108-5.html

(see "Cached Page Load Time" benchmark)

Of course these don't precisely measure cache perf, since they
encompass all the logic to load and display the pages.   (They
presumably also are results using a fairly empty cache, which might or
might not affect perf).

The take-away would seem to be that we're not doing anything really
horrible (yay), and are competitive with IE/Opera, but we could
improve somewhat (Chrome tends to be faster).  Nothing earth-
shattering or market-defining, IMO.