the CPU usage of glReadPixels() and glTexSubImage2D()

Does anyone know the CPU usage of these two API?I do a test in my computer(2.4G+ATi Radeo9800SE+DDR 1G),I found the glTexSubImage2D() will consume about 10% CPU usage(about 240M),and the glReadPixels() consume about 25%(600M),so I want to know is your computer same with my result?I also hope you post your test result.