When the "main" and "guest" networks are enabled, the available bandwidth is split between these two networks. So, performance wise....it would be a good idea to only enable the "guest" network when you really have guests.
I am assuming here that you have installed the latest Lion updates....including AirPort Utility 6.0.
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme icon
Click the Wireless tab
Remove the check mark next to Enable Guest Network
Click Update and allow 25-30 seconds for the AirPort Exteme to restart
Reverse the procedure above when you want to enable the "guest" network again.
The Guest Network feature of the AirPort Extreme is a form of VLAN implementation. As such, this network will need to share the overall bandwidth available with the "main" network ... so, yes, running both networks will reduce the overall bandwidth for both.
The Guest Network is NOT enabled by default. If you or someone else has already enabled it, it is easy to disable using the AirPort Utility as follows: AirPort Utiilty (v5.x) > Select the Extreme > Manual Setup > AirPort > Guest Network tab > Enable guest network (unchecked)