This has always been a kind of no brainer but a recent study showed WiFi connections on average were 30% slower when compared to connections with a wired setup.

An article posted today on BBC News indicates over a million tests were conducted on some 14,000 wifi connections revealing the slowdown.  An interesting point in the article is the fact most people are content with the speed reduction in exchange for the wireless mobility.  The original article can be found here BBC News.