eclipse - Alpha blending is slow on linux -

We are developing a SWT / RCP (Eclipse 3.5) application on both Windows and Linux (on similar hardware). This application is a GIS app that shows many layered maps (PNG images) provided with alpha blending.

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

The display on Windows is quite good, but it is very bad on Linux.

Is this a Linux (GTK / KDE) problem? Or is there any way to improve performance on Linux?