What are the best available D (not C) File input/output options?

[confuzzled]

I've ported a small script from C to D. The original C version takes 
roughly 6.5 minutes to parse a 12G file while the port originally took 
about 48 minutes. My naïve attempt to improve the situation pushed it 
over an hour and 15 minutes. However, replacing std.stdio:File with 
core.stdc.stdio:FILE* and changing my output code in this latest version 
from:

	outputFile.writefln("%c\t%u\t%u\t%d.%09u\t%c", ...)

to
	fprintf(outputFile, "%c,%u,%u,%llu.%09llu,%c\n", ...)

reduced the processing time to roughly 7.5 minutes. Why is 
File.writefln() so appallingly slow? Is there a better D alternative?

I tried std.io but write() only outputs ubyte[] while I'm trying to 
output text so I abandoned idea early. Now that I've got the program 
execution time within an acceptable range, I tried replacing 
core.stdc.fread() with std.io.read() but that increased the time to 24 
minutes. Now I'm starting to think there is something seriously wrong 
with my understanding of how to use D correctly because there's no way 
D's input/output capabilities can suck so bad in comparison to C's.