Alpha blending is slow on linux

we are developing an SWT/RCP(Eclipse 3.5) application on both Windows and Linux (on identical hardware). The application is a GIS app which shows several layered maps(PNG images) rendered with alpha blending.

org.eclipse.draw2d.Graphics.setAlpha(...);
org.eclipse.draw2d.Graphics.drawImage(...);

On Windows the performance is pretty good, but on Linux it is very poor.

is that a Linux(开发者_如何学PythonGTK/KDE) problem? or is there any workaround to improve the performance on Linux?