Technically, adding RAM does not speed up anything, it *removes impediments to the machine's speed*
Like saying: "Taking your foot off the brake makes your car speed up", no, taking foot off brake just stops slowing it down.
The first reason to add RAM is to avoid the OS doing massive amounts of swapping to the hard drive when your OS Program and Data load exceeds the amount of physical RAM. Since hard drives are WAAAY slower than RAM, when the machine has to page out to disk, it slows down all operations of the machine. Adding RAM in this case will improve this situation generally, on all programs running.
So if your Web pages were slow because there wasn't enough real memory to run the Flash and Javascript and what have you without paging out, then adding RAM would restore the Web browser to its potential speed.
Thanks
Trevor
CanadaRAM.com