bug#71591: 30.0.50; PGTK: Terrible input lag with display scaling

[Po Lu]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Jarno Malmari <jmalmari@ftml.net>
>> Date: Sun, 16 Jun 2024 15:26:13 +0300
>> 
>> With a 4k display and 150 % display scaling there is a noticeable input
>> lag when moving the point around in fullscreen. Very sluggish
>> feel. Shrinking the window helps.
>> 
>> Besides "sluggish feel" I couldn't figure out easily measurable metric
>> for this. One way is to clock how long it takes to use your keyboard
>> repeat with `next-line' to reach the end of a couple pages long buffer.
>> 
>> This was tested with PGTK build on KDE Plasma 6 running Wayland. X build
>> running in XWayland does not have this problem even if configured to be
>> "Scaled by the system" from the system display configuration UI.
>> 
>> The scale option that affects this can also be changed with these commands:
>> 
>>     kscreen-doctor output.${OUTPUT_NAME}.scale.1.5
>>     kscreen-doctor output.${OUTPUT_NAME}.scale.1.0
>> 
>> Applies also to emacs 29.
>
> Po Lu, any suggestions or ideas?

Jarno, does your system use NVIDIA graphics drivers?