Wifi speed: Desktop/laptop > iPad > iPhone?
iPhone 4S has a smaller antenna and slower processor (A5 vs. A5X) than iPad 3rd generation. More importantly, the iPhone 4S wi-fi is (802.11 b/g/n) 2.4 GHz only, while the third generation iPad has two channels — 802.11a/b/g/n Wi-Fi (802.11n 2.4GHz and 5GHz.)

I don't think the iPad uses both channels at once, but I think if the second channel is more clear or faster to your router, it would not be surprising the iPad gets faster speeds over Wi-Fi. I think a faster processor alone would allow for faster Wi-Fi speeds.

Similarly, the faster CPUs in MacBooks and iMacs would allow for faster Wi-Fi speeds, even if the networking architecture is the same. (I suspect it isn't, and that there are larger antennas in Macs than iPads, which is another factor…)