GPIO speed and GPIO reversal speed

Hi,
I bought two upboards, the operation system is ubilinux, I use eclipse to develop my program.
I have DAC chips that need the GPIO reversal speed is more than 800 kHz to control the sampling rate. But I tested today that, the reversal speed is about 90 kHz. The test code is example from /usr/share/marr/gpio-blink.cpp . The main code is
iopin=8;
signal(SIGINT, sig_handler);
mraa::Gpio gpio(iopin);
mraa::Result response = gpio.dir(mraa::DIR_OUT);
if (response != mraa::SUCCESS) {
mraa::printError(response);
return 1;
}

while (running == 0) {
response = gpio.write(1);
sleep(1);
response = gpio.write(0);
sleep(1);
}
return response;


I connect iopin=8 to the oscilloscope, I can see the period is about 2s, it demonstrates that the GPIO woks well.

then I replace the while loop as
while (running == 0) {
response = gpio.write(1);
response = gpio.write(0);
}

the delay function has been deleted, I check the oscilloscope, the period is about 11us. I think the reversal speed is quite low.
Do you have any idea to improve the reversal speed.
Another question, I saw the upboard supported high resolution timer at 1 ns, do you have some examples to use the high resolution timer.
Thank you very much.
Best regards
Ruddy