bug#23928: 25.0.95; Performance regression observable with smartparens

[Eli Zaretskii]

> Cc: aaronjensen@gmail.com, npostavs@users.sourceforge.net,
>  23928@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Sun, 10 Jul 2016 22:17:31 +0300
> 
> I have to say that the idea of using the above for changing the value of 
> case-fold-search instead of let, is surprising for me. Most of the time, 
> one is using let just for that effect.

It only causes trouble if done in the innermost loops of your
program.  Otherwise, the cost is almost negligible.