But your distinction between a plain text file and the .html output of your OP puzzles me again, as any difference should be negligible: the .html source is, to all intents and purposes as far as writing the file is concerned, a "plain text file."

Or does your actual script open a browser after writing the output? That, of course, would add a bit more overhead. In that case, you cantrim the rendering time by assigning width to table and the ten tds.

OTOH, it may be even more useful to await better answers from Tk gurus here.