>>>>> On Thu Jan  2, Richard writes:

>> This issue is now closed.  I discovered that the implementation of
>> "wide-column.el" was the culprit.  It was abusing the
>> post-command-hook in a way that worked in 21.2, but not in 21.3.
>> I have referred the problem to the author.

> Can you tell me what the problem is?  Maybe I should fix what
> broke.

I did not take the time to track it down, nor can I just yet.  I
would recommend talking with the author, as likely he will be
interested in fixing it to be compatible with 21.3.  His e-mail is:

  Phillip Lord <p.lord@russet.org.uk>

To reproduce, just load the module in 21.3 and start using it during
your regular workday.  The slow-down should become apparent.