read_colophon is several milliseconds of computation over a few hundred
kilobytes, and it held the GIL for all of them. A caller reading a
library through asyncio.to_thread therefore got no overlap whatsoever:
six books in flight, one book's worth of work at a time.
Measured on this library, that was most of why a concurrent pass still
paced at four and a half hours.
funkpower
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