They use some form of cache or compression in one way or another, like that evil google accelerator program! It caches pages from links on the page your looking at, so it comes up quicker... Only downside is that it caches a whoooole lot more than what you would look up, mainly chewing up bandwidth and if you have a download cap, it might go over faster than expected. In a delightful manner of answering your question. No, they don't increase it. Only hinder it.