All visual G4P controls have their own off-screen buffer to store their visual interface. This buffer is lazily updated so that drawing the control in the display is a two step process
- Update the buffer image if and only if the control’s state has changed since the last frame
- Copy the buffer image to the correct position position in the display
This is very efficient compared to redrawing the control from scratch every frame even if there is no change
When a sketch is launched all the control buffers are invalid so have to be created hence the small delay. So sorry there is no realistic alternative to this delay. 
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It would be useful to know what versions of Processing, G4P and GUI Builder you are using.
I am not aware of any stability issues with the current version of GUI Builder. It would be foolish to claim that GUI Builder has no bugs but a program crashes when it hits a situation it can’t deal with. Try and think what you were doing when it crashed e.g. editing a control, scrolling the GUI Builder window …
One thing I recommend when using GUI Builder do not have multiple sketches open at tha same time, just GUI Builder and its associated sketch.