RL-FlashFS performance

Hello,
Can anybody show me how to quickly write large files to the SD card using RL-FlashFS?
I did a simple test, but recording speed is too small for my tasks 1MB/s:
printf("time start: %d\n", ms_counter);

unsigned char count[512];
FILE *f;
f = fopen ("benchmark.log", "w");
for(i = 0; i < 40960; i++)
{ fwrite(&count,1,512,f);
} fclose(f); //
printf("time end: %d\n", ms_counter);

Keil Technical Data for RTX Flash File System for my LPC2468 4 MB/s (http://www.keil.com/support/man/docs/rlarm/rlarm_fs_technical_data.htm)

Thanks in advance..