Created attachment 132626 [details]
images to compare the problem with lag in pattern feature

SUMMARY
Brush editor suffer lacks while editing parameters if we use a normal RGB pattern. In my desktop with Win10 and i7 the elag is like 0.5 seconds. a bit. But in the laptop with Kubuntu last update october 2020 and i5 CPU The lag is arround 3-4 seconds. 
Only happens in brush editor while pattern is active. If we deactivate pattern feature the brush editor works perfectly again. 
The problem also dissapear if we use greyscale png images. I give link to 2 images as reference. 
Maybe is this maybe i am doing something wrong with RGB saving? let me know please.

The default Patterns works ok. this lag happens when you copy the png files to a pattern folder

STEPS TO REPRODUCE
1. Import two patterns included RGB and Greyscale png patterns. or Copy directly to the pattern folder
https://drive.google.com/drive/folders/1_g_VSbLhGUND7xre1BmXKv-BEnp2LPeH?usp=sharing
 
2. Open Krita if it is not open 
3. Activate the pattern feature and compare with the 2 imported patterns.

OBSERVED RESULT
lag with RGB png file
no lag with greyscale png file

EXPECTED RESULT


SOFTWARE/OS VERSIONS
Windows:10 
macOS: 
Linux/KDE Plasma:Kubuntu 2020 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Tested in 4.3 and 4.4. Same result.