Gradius,
By default there is a 4-second delay in answering the incoming analog PSTN Line call to capture the incoming caller id. If you don't wish the incoming caller id you can remove the delay by changing the Line Port-->RingDelay setting to 0. If you are doing selective forwarding based on the caller id then, of course, you need the incoming caller id and the setting should not be changed. This change would allow a small reduction in the voip call setup time. The time to setup the outgoing call can also vary depending on the voip provider you are using.
If you are forwarding the call using GoogleVoice, GoogleVoice as a matter of policy does not show any caller id other than your GoogleVoice Number. If caller id is important to you, you need to do the forwarding with a sip account but even when using a sip account for the forwarding it is usually complicated to pass the incoming caller id.