It's all tied to the speed and efficiency of the proxy. Some hardware is more efficient, some software is more efficient. How much other activity is going on in that box, etc also makes a difference. Generally speaking, it's a compromise, and "connection shareing" will be much slower than a dedicated router/FW (ala LinkSys, SMC, etc).
A specific value (like a performance ratio) can't be determined because of the spectrum of possible computers, software, activity levels, cabling, ISPs and other factors is so broad. Generally speaking, a dedicated, built-for-purpose box is gonna outperform an adapted PC, especially if the PC is running an MS product (LINUX would be faster on the same platform). If I had to throw a dart....I'd guess that a LinkSys (SMC, D-link, ...) router is probably in the neighborhood of 15-30% faster/more efficient than a PC with Windows...but again, that's just me pulling a number out of a dark place.....
FWIW
Scott