What could cause fwrite() function so slow

What could cause fwrite() function so slow

I'm writing chucks of chat *ptr into file, but it takes me so long to
finish. I don't know why.
for(int i = 1; i < 64048; i++){
if(ptr[i] != NULL){
fwrite(ptr[i], 1, strlen(ptr[i])-6, file);
free(ptr[i]);
}
}
there is array of char pointers storing 8186 bytes, and I want to write
into file, but the time is so long.
I have 64048 chunks of string need to write into file.
What could cause the slowing fwrite() ??
thank you