https://youtu.be/3Q46Tp8kBaQ
This is Krita 4.2 on Huion GT-221 PRO. The Lag can be seen clearly on Triangle Testing.
PC Spec : Windows 10, i7-4xxx, Nvidia GT 730

https://youtu.be/peRF_A19hM4
This is Krita 4.2 and Photoshop side by side testing. Wacom Intuos Draw. The brush looks like snappy and not smooth compare to Photoshop.
All setting Brush Smoothing to Basics
Most Lagging happens when you starts the stroke. It lags from a while and then jumps to where the pen is.
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Edited
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Tips : Try using Angle instead of Opengl in setting>Display. Dont forget to restart

Hi,
i have a similar issue but after some times of use, in my case. The problem won't occure if i close and open again the file, but appear again after a certain time. i tried differents things opengl/directAngle... i think i've found that it's a problem with smoothing, i set it to none and i could paint for hours without issues

Oh, that's a useful hint!

@boudewijn i've noticed this problem on the beta version too but i couldn't found the origin of the problem.
as it happens after some time, I though about autosave at the beginning, but the problem happen with autosave disabled.
i haven't notice before but the basics smoothing seems to be activated by default in the 4.2 (i'm not sure), and it look like this is causing the issue...

smoothing set to none appears to work in my case, i'll test it more this weekend

so, i've tested a little more and the lags come again after a while (with smoothing set to none)... as it's the cursor that is lagging, i go to Settings / general / cursor and untick "while painting... use effective outline size" and the brush stroke get nice and smooth without having to close the file. I even reactivate the basics smoothing and it's ok for half an hour now...

hope this can be usefull...

Does that lag disappear when you switch to another renderer? I've noticed some lag after a while (+30 minutes into a document) and switching to Angle from OpenGL or vice versa would eliminate that lag, until it manisfests itself again.

@Gremiel, no i haven't test it, because when you change the renderer mode, you have to restart Krita, but i keep the option in mind, i'm testing

@mcoudert I don't have to restart Krita, just switching form OpenGL to Angle or vice versa removes the lag for me.

oh ok, i was convinced that you have to restart Krita to validate this change. That can be another hint for the devs

Here i'm again. First, yesturday i painted on the afternoon and in the evening = no problem. Today, the lag come after a while, i just go to settings... use check "effective outline size", return to canvas, then un-check the option again and the problem was solve. I've been painting for 2 hours without any lag now.

On the social networks i follow, i've seen other users complaining about this lag, they have differents solutions like desactivating canvas graphic acceleration, switch to opengl or direct3D... it look like if chaging an option in settings re-init something.

@boudewijn It is difficult to identify the problem, so i want to know if the tests i've made are usefull for devs? is there a way to be more effective (logs, ideas on what to look for)

Yes, this is a useful observation. Please add it to https://bugs.kde.org/show_bug.cgi?id=408133

@boud from my testing, krita-nightly-x64-v4.2.0-HDR-307-g96b04eda2b is the last version that does not have this lag issue.

Cool, can you please also add that to the bug report?

I try it again with Huion on my PC with " Direct3d with ANGLE ". (Restarts Krita)(Thanks for Tips)
It makes the brush smooth and solves snappy issues.

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BTW, here is kind of loading when clearing canvas. I don't mind this thing actually.
https://www.youtube.com/watch?v=nsTyFe3bTk0
But this is what I feels when i am drawing. The brush is kinda loading like this thing clearing on Canvas.
Although the lag improves a lot when using ANGLE, I still feels this kind of feeling. I cant explain well. The Brush Stroke just flashes and loads quickly.
But I know the feeling is a little off compare to CLIP or Photoshop.

Hello everyone, I noticed the same problem: after some time spent painting on the same file the brushes become laggy. It all resolves just by closing and opening the file again, with no need to close krita.

I tried some troubleshooting without much success, then I noticed something that I hope may be of use: my "undo stack size" is set to 20 in the settings (default I think?), but krita is somehow keeping all of the changes I make from the moment I open the current file in my undo history. I tried setting a different value (15), and it works until I close krita, but after reopening the program the effective undo size goes back to infinite, while the option in settings still is set to 15.
I dont' know if this is significant, but it was not happening before 4.2, and I thought it may perhaps give one more clue about this problem.

Thank you.