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.
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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.

See [How do I read and write files in Tcl].