I/O library 6x faster than fmt, 10x faster than stdio.h and iostream
In many applications I can get 10x faster I/O using stdio.h  just by radically increasing the buffer size. It turns out that less you talk to the kernel, faster you are.     /*  typically 16 to 32 times  
         stat.st_blksize  or BUFSIZ 
         is good */

     int new_size = 16 * 4096;

     setvbuf(fp, NULL, _IOFBF, new_size);