A format string now gets parsed only once instead of re-parsing it each
time it is applied to a set of data.
The initial parsing causes directory path creation to be at about 2x
slower than before, since each format string there is used only once,
but building a filename, the more common operation, is at least 2x
faster. The "directory slowness" cancels at about 5 filenames and
everything above that is significantly faster.
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