LPC2148 is slow in Realview

Dear all,
I am using Realview (evaluation) and have the following program for LPC2148:

#include <LPC214x.h>
#define LED 0x01000000
main() {
        IO1DIR |= LED;  //use P1.24
        for(;;){
          IO1CLR |= LED ;
          IO1SET |= LED;
  }
}

the crystal is 12M and multiple factor is 5, no VPB setup (i also tried VPBDIV=1). It means the system should run with CPU clock=60Mhz
The problem is the LED blinks only with 1 MHz.
I tried simulation but the time shown there still indicate that the LED blinks with frequency around 30Mhz.
This happen not only with the LED but with any code i put. It seems that my system runs with 2Mhz CPU clock. Does any one have the same problem, please help me. Thanks a lot.