bug#21028: Performance regression in revision af1a69f4d17a482c359d98c00ef86fac835b5fac (Apr 2014).

[Eli Zaretskii]

> Date: Fri, 10 Jul 2015 03:34:23 -0700
> From: Clément Pit--Claudel
>  <clement.pitclaudel@live.com>
> 
> 2. Run the following code:
>     (progn
>      (dotimes (_ 3)
>        (set-fontset-font t 'unicode (font-spec :name "Arial") nil 'append))
>      (dotimes (_ 50)  
>        (insert "日本国\n")))
> 3. Move the cursor around. Notice that motion is extremely slow (in 
> particular C-p and C-n). So is shift-selection. (if it is not, try increasing 
> the 3 above, maybe to 5 or 10).
> 
> This test case is the smallest I could come up with; there might be other 
> ways to trigger the problem. In my particular case, it is caused by having 10 
> or so distinct fontset rules, covering different blocks of characters. This 
> problem was mentioned elsewhere online too (for example 
> https://github.com/purcell/emacs.d/issues/273).

What kind of complex fontset setup do you have?  FWIW, I don't see any
perceptible slowdown with your recipe and the default fontsets.