I fond some reasons for the slow graphic. With Subpixel Hinting turned off and with a theme that is not based on murrine or clearlooks, gtkperf runs much faster. less than 30 seconds for the same test.
With new RAM (1GB DDR 133MHz instead of 512MB SDR 100MHz) and a new gfx card (Gforce 6200), without subpixle hinting and Mist as theme gtkperf flys through the test in less than 20 seconds. An amazing performance boost for 25€