Wireless are prone to slow DL speeds. Especially if there is a lot of noise or a weak signal, it can cause large numbers of transmission errors which result in wasted time and thus less time for new data to be transmited.
Try changing the frequency of your wireless to reduce possibilities of noise. If you are gaming, throw the wireless away and get wired. Although wireless gamming is possiable, it generally is unadvisable as not only can the connection be unreliable, but also it has higher ping which is very bad in gaming.
If your service is comming via cheap ADSL or other such services of low price, it is almost certainly oversold and so when everyone uses it insanly slow speeds are to be expected.