Severity: wishlist

The below patch makes asynchronous fontifying of man pages faster.

After typing `M-x man RET gcc-14 RET`, it takes around 5 minutes on this
fast MacBook Pro M2 (2022) machine until the whole 1.43 MiB man page is
displayed.  With a hot cache, the best time I have managed to measure is
173.432 seconds.

We seem to call `man--maybe-fontify-manpage` many times on very small
chunks: on my machine, it processes ~30 characters at a time.  Things
get substantially faster if we make `Man-bgproc-filter` fontify the
buffer in much larger chunks.  The main drawback of doing this is that
we risk very briefly seeing an incorrect display flash by in the man
buffer (e.g., with `end-of-buffer`).

My measurements show that 32 KiB might be a good choice, and gives a
~95% speedup:

| Chunk size (KiB) | Completion time (s) |
|------------------+---------------------|
|                4 |                29.3 |
|                8 |                18.2 |
|               16 |                12.7 |
|               32 |                 7.4 |
|               64 |                 6.5 |
|              128 |                 5.4 |

Is this the best approach to optimize something like this, or am I
overlooking something obvious?  Note that I didn't add a variable for
the chunk size, but we could easily add one, if that'd be useful.

When I set `Man-prefer-synchronous-call` to t, it takes ~1.8 seconds to
process the same page.  I guess that this is the lower bound for how
fast we could make the asynchronous call.

Please see the attached.