how fast does AGP Bus read data?

my program need to read data from display card,I use “glReadPixels()” to do it,but I found it only read 2M-3M bytes per second,the speed is too slow than my expectation,so how fast does AGP Bus read data?my test is correct?

http://www.opengl.org/discussion_boards/ubb/Forum3/HTML/008155.html