The speed of a WiFi connection is determined, in TCP protocol, by the fact that, most often, packets have to be re-transmitted (in UDP, dropped packets are simply forgotten). The reasons why a packet might be dropped are, on the one hand, a weak signal, and on the other hand the presence of other wifi networks (or other radiation, see later) on the same or a nearby channel. This interference by other channels is often overlooked as a disturbance factor, but especially for antennas like the Alphas mentioned above, which operate at 2.4GHz exclusively, in crowded areas it has become the major limiting factor to wifi speeds.
Besides other wifi networks, there are many other sources of radio emission, on the 2.4GHz channels, which are often unrelated to computers. You can find on this very forum an excellent discussion of this phenomenon. Common sources are cordless phones, cell phones, microwave ovens, and sundry electrical home appliances.
If your line is very slow, you should keep this in mind, and try to position your antenna and router as far as possible from objects that may cause interference.