SDHC card interface, write 512 byte time

Anyone who has interfaced SDHC.

How much time it will take to send a frame of 512 bytes with 18Mhz speed over SPI protocol.

Theoretically it will be 18Mbits/sec i.e 227us for 512 bytesr.

But my code is taking 750us. Am I mistaking somewhere:

uint32_t i;

for(i = 0; i< 512 ; i++)

{

send_byte(my_array[i]);

}

inline uint8_t send_byte( uint8_t  temp )

{

   while (tx_buffer_not_empty or busy);

DATA_REG = temp;

while(rx_buffer_empty or busy);

returrn (DATA_REG);

}