In a c.l.t posting, Antonio D'Aprile recounts that WinNT disk writes can appear much slower than screen writes. The symptom vanishes, though, with a bit of buffering: fconfigure $out -buffering full -buffersize 8192 Antonio says the "full" matters more than the specific buffersize.
It's long been part of the Tcl folklore that expansion of the I/O buffer accelerates disk *reads*. CL doesn't know whether this remains true with 8.3.
Sounds like this discussion may be related to recent flush improvements released in Tcl 8.3.4. See http://sf.net/tracker/?group_id=10894&atid=110894&func=detail&aid=219148
See How do I read and write files in Tcl.