The speed of encryption is highly dependant on the processor/chipset used in your router. I would expect that any bandwidth tests on type of encryption would be per-router, I would also expect more modern routers to use hardware encryption and decryption to mitigate any performance loss.
Sure no encryption at all will be fastest, after all there is nothing to do but transmit data.
After that WEP would be faster than WPA/WPA2, but WEP is almost as good as having no encryption at all as far as security is concerned. As noted here a WEP protected network "can be cracked in 10 minutes easily." While you can use a VPN to secure your traffic you might as well just invite everyone in the neighbourhood to use your WiFi.
In all honesty if you get a newer or higher end router router I would expect the performance loss due to encryption to be minimal compared to the speed loss due to signal noise.