Answer taken from speedguide.net.
Question: Does WEP/WPA slow down wireless connections ?
Answer: Yes, WEP and WPA encryption add some overhead in terms of calculations needed to encrypt/decrypt the traffic. The exact impact greatly depends on the processing power of the network device, it can vary from 5% to 30% of the maximum throughput.
With newer routers that have faster processors, enabling WEP/WPA should not have a major impacts on performance, it should be barely noticeable at full load. Some older models, however, can slow down as much as 30% or more due to the added processing overhead.
Even though there is some perfomance impact, it is not noticeable in newer devices, and the benefits (security) far outweigh any drawbacks.
Note that WPA2 (AES), in addition to being more secure is also much more efficient than WEP/WPA(TKIP) and should be used, if possible.