bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 02 Sep 2024 10:18:03 +0100
> From: "Stephane Travostino" <sph@combo.cc>
> 
> Heavy operations, such as scrolling back and forth in a buffer, are
> noticeably laggier, for lack of better word, in the PGTK/Wayland version
> than the X11, both tested on KDE in Wayland mode. 
> 
> Affects both 29.2 and the latest HEAD compiled a few days ago.
> 
> I am unsure whether it is a KDE or Emacs problem.
> 
> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
> scaling.

AFAIU, this is a problem with GTK input methods.  From PROBLEMS:

  *** Emacs built with GTK lags in its response to keyboard input.
  This can happen when input methods are used.  It happens because Emacs
  behaves in an unconventional way with respect to GTK input methods: it
  registers to receive keyboard input as unprocessed key events with
  metadata (as opposed to receiving them as text strings).  Most GTK
  programs use the latter approach, so some modern input methods have
  bugs and misbehave when faced with the way Emacs does it.

  A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
  find a different input method without these problems.