Performance/QoS implications of Secure S

Hi, all

I am new to VoIP and SIP. I have been wondering about the
performance/QoS implications of secure SIP. By securing
SIP, the client has to do a lot of extra cryptograhic operations,
for example encrypting every VoIP packet, will that degrade
the quality of the call? Especially quality is critical for VoIP and
the phone clients may not be very powerful. Or that is not a
problem at all and the QoS issues for SIP mainly come from
the network?

Could somebody point me to some related resources?

Thanks so much

Tao