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.