There should be a checkbox allowing to enable/disable preview and its state should be saved in the preferences file
As soon you launch gtk-recordMyDesktop, theres too much X11 lag because of the preview
even with core2 duo T7700 @ 2.4ghz (at 1440x900 resolution)
I never select a zone in the preview, I always use selecting window (and it is boring because of the lag)
After that maybe i have to do some operations in the window before starting record and here again it is boring because of the X11 lag.