The Wi-Fi chip and the CPU can only process so much bandwidth at once. On older handsets you are probably going to see slower speeds with regards to bandwidth because of what I described above.
For example, my old Sprint HTC Hero, which is an ARMv6 CPU running at 528 MHz when not overclocked, usually pulls down a maximum of around 5 to 6 Mbps on Wi-Fi, whereas my Sprint HTC EVO 3D, which is a dual-core ARMv7 CPU running at 1.2 GHz when not overclocked, can easily pull down much more than double that on Wi-Fi. It is all about what your device's Wi-Fi chip and CPU can handle bandwidth wise. Signal strength plays a large part in the equation as well. Bandwidth speeds can be greatly reduced if there is a lot of interference or you are very far away from the Wi-Fi router. Just food for thought. I hope this helps.