Too many folders slows imapsync

[Mark <markauk at gmail dot com>]

I have been migrating an archive of old mail to my gmail account, and now
have nearly 1200 labels in my gmail account. I want to consolidate these
into a few labels, but unfortunately Google doesn't allow wildcards to
search label names, and the search string has a fairly short length limit so
I can't easily get Google to display messages from a lot of folders in one
view (in which case I could easily then apply a new label to those messages
and I'd be done).
imapsync to the rescue!

I have written a script to run imapsync successively, moving messages from
once Gmail folder to a new one. This works fine, but with one catch. It is
taking about 25 minutes to run once (even if it's only copying a couple of
small messages). Before I had all these labels, imapsync was running
reasonably quickly (maybe a couple of minutes per run).