In Delphi 2009 I found that the flicker of a PageControl - which occurs during resizing of the form - can be reduced by setting its DoubleBuffered property to true.
However if I add controls to the PageControl tabsheets, they will flicker regardless of their DoubleBuffered property setting. I have also tried with and without runtime themes enabled.
Setting ParentBackground to False for components on the PageControl helped a lot. However this results in a different color of these panel components, they all have a darker background now. Maybe this can be fixed easily (without losing Theme support).
I also installed VCL Fix Pack which has a fix for QC 56252 (TPageControl flickers a lot with active theming).