bug#38407: 27.0.50; infinite loop with display of large file without newlines

[Eli Zaretskii]

> Date: Thu, 28 Nov 2019 19:30:50 +0100
> From: Pieter van Oostrum <pieter@vanoostrum.org>
> Cc: psainty@orcon.net.nz,
>     38407@debbugs.gnu.org
> 
> In Emacs 26, it is also slow, but the redisplay comes to an end, so I see 
> this as a kind of regression. The timing with Emacs 26 is very similar to 
> that in Emacs 27, except that it doesn't hang at the end. So my impression is 
> that the real difference is when the scrolling reaches the end of the file. 
> That also happens with M-> (end-of-buffer), which hangs indefinitely (or at 
> least a long time that is virtually indistinguishable from indefinitely) in 
> Emacs 27, but it does finish in Emacs 26.

I don't think there's any difference in this aspect between Emacs 26
and 27.  I see the same problem in both, FWIW, and I definitely didn't
wait for hours for redisplay to finish, let alone saw it "hang
indefinitely", even in an unoptimized build of Emacs 27 on a very slow
machine.  In my case it took something like 15 minutes.  I don't
understand why you see such long times.  Of course, I tried this with
a different JSON file than you did, so maybe there's something in your
file that causes even more slowdown.

In any case, try the latest master after setting bidi-inhibit-bpa to
non-nil.  In my testing, this is as fast as Emacs 23, i.e. no bidi
reordering at all, give or take some 10%.

And if you want the fastest display possible, use Fundamental mode,
not Text.