Displaying a sorted virtual buffer is very slow

[Michael Ernst]

I find Mew's sorting very slow.
The problem is not the sorting itself, but the rearrangement of summary
lines in the virtual buffer.
For a buffer with 11,000 messages, that takes 2.5 minutes on a recent,
fast machine.

My investigation indicates that mew-summary-selection-by-sort-body calls:
* mew-sort-get-file-index which calls sort internally and is fast. The
second element of its result (named idx) appears to be the sorted outcome.
* mew-sort-files which is slow. Despite its name, it doesn't actually
do any sorting. It creates a virtual folder with the messages in original
order, then it has a doubly-nested loop that rearranges the buffer into
sorted order.

Creating and displaying a threaded virtual buffer for a large mail folder
is fast, so there is not a fundamental problem that makes displaying a
reordered virtual buffer slow.

Does anyone have ideas about making the display of the sorted results
significantly faster?

Thanks,

-Mike