Actually, I'm guessing their software does some combination of a) skipping
part of the IP stack, b) caching, and c) compression. There could also be
static content (known ads, backgrounds, scripts, etc.) that is accessed
locally instead of fetched over the modem, and perhaps some sort of
filtering. Also, there could be some user experience management going
on, where it loads the stuff you can see, and gets the rest of it while
you're reading the first part (something real browsers have been doing
for a long time).
 
There could also be static content (known ads, backgrounds, scripts, etc.) that is accessed locally instead of fetched over the modem
I guess those would be the same ads I don't retrieve at all, thanks to an ad- (and pop-up)-blocking proxy (privoxy, but there's others). That really sped up my dial-up surfing.

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