Wireless routers that are changed from no security to WPA are not as quick. Is there a reason for this?

Of course it will be slower using WPA, since everything you send and recieve will be encrypted.  But for me, as a human, I cannot tell the difference.  If you hook some gaget to your connection and counted the nanoseconds you would probably be floored on how slow the connection is, but for me, I can't tell the differance.  

For PCs, the difference is not noticed; as for other devices such as PDAs or smart phones, you might feel some difference when using VOIP or Video applications.

I had one smart phone configured using WPA2 and voice was a little choppy compared with WEP; I would assume with no security voice will be even better; but Security is not something you should take lightly + this smart phone is a 2 year old model phone. The processors on the new phones and PDAs most likely will not be as bad.

The reason for the slowness is because each packet gets encrypted and decrypted which is a mathematical process which can slow down the conversion of each packet as opposed to sending the packets without this process.

WPA-PSK encryption is technically less overhead than WEP.
WPA uses the passphrase for initial authentication, then switches to encryption (based on the passphrase) instead of sending the key along with every packet, which is what makes WEP so weak.

That was interesting reading, but Ive seen 5 bars drop to three on several different routers in several different locations so there is some factor at play.

The security used definitely does not affect signal strength; there must be something else 'at play' there affecting it. Unless whatever you're using to measure throughput gives its rating in '0-5 bars' instead of bits per second.

If it was at 5 bars and then after changing to WPA its 3 bars  apart from that, how do you suggest measuring throughput?

http://www.speedguide.net/speedtest/
for one.

Try changing your frequency channel on the wireless router to either 1, 6 or 11 and see if the strength of the signal improves.

I think the question has already been answered with many perspectives on this issue.

The question was why WPA is slower than no security at all - and many answers were given.