qt 4.8 slower with raster graphicsystem on Linux

I read at https://bugreports.qt-project.org/browse/QTBUG-19636?focusedCommentId=177704 that default graphicssytem is on qt 4.8 raster, instead of native, because it offers better performance.
However, if I run http://code.google.com/p/qtperf/ on qt4.8.1 with default graphicsystem (raster), it works 4 times slower than with native graphicsystem (which then gets similar performance than with qt 4.7), even with local X11. I have an i7 with nvidia GeForge GTS 250.

thanks

./qtperf4 -graphicssystem native
QLineEdit - 0.097 s
QComboBox - 0.358 s
QComboBoxEntry - 0.389 s
QSpinBox - 0.025 s
QProgressBar - 0.428 s
QPushButton - 0.056 s
QCheckbox - 0.017 s
QRadioButton - 0.039 s
QTextEdit add text - 0.689 s
QTextEdit scroll - 0.381 s
QPainter lines - 6.34 s
QPainter circles - 6.267 s
QPainter text - 0.805 s
QPainter pixmap - 0.056 s
Total: 15.947001 s

./qtperf4 -graphicssystem raster
QLineEdit - 0.085 s
QComboBox - 0.563 s
QComboBoxEntry - 0.485 s
QSpinBox - 0.026 s
QProgressBar - 0.431 s
QPushButton - 0.069 s
QCheckbox - 0.031 s
QRadioButton - 0.065 s
QTextEdit add text - 0.45 s
QTextEdit scroll - 0.247 s
QPainter lines - 26.022 s
QPainter circles - 27.663 s
QPainter text - 3.071 s
QPainter pixmap - 0.294 s
Total: 59.502000 s