Bug 437950

SUMMARY
When a huge pattern is selected in Texture->Pattern tab in brush editor, the rest of the editor is very very slow. Most probably because either there is no signal compressor, or even with the signal compressor the rate of repainting a stroke is too high for Krita to be able to paint it.
Note that the same brush doesn't really have trouble painting on the canvas in small sizes.

STEPS TO REPRODUCE
1. Download Flow's bundle 1.0 from here: https://krita-artists.org/t/flows-mixer-concept-brushes-v1-1-work-in-progress-krita-5/24304 . If it cannot be downloaded, any big pattern (1024x1024, or maybe 2048x2048) in .jpg format (not sure if format is important) will do.
2. Select f) Bristles-3 Large Smooth.
3. Confirm you can change the Spacing in Brush Tip tab easily.
4. Select "TheFlow Canvas 01".


OBSERVED RESULT
All sliders are super slow now. See Brush Tip -> Spacing, for example.
If you close the editor (with still usually high Spacing) and start painting, you'll see that the brush is painting with usual speed (very responsive).

EXPECTED RESULT
All sliders are still fast and responsive.

SOFTWARE/OS VERSIONS
Krita

 Version: 5.0.0-prealpha (git 626af155fb)
 Languages: pl, pl_PL, pl
 Hidpi: true

Qt

  Version (compiled): 5.12.8
  Version (loaded): 5.12.8

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 5.4.0-58-generic
  Pretty Productname: Linux Mint 20.1
  Product Type: linuxmint
  Product Version: 20.1
  Desktop: X-Cinnamon

OpenGL Info
 
  Vendor:  "Intel" 
  Renderer:  "Mesa Intel(R) Iris(R) Plus Graphics (ICL GT2)" 
  Version:  "4.0 (Compatibility Profile) Mesa 20.2.6" 
  Shading language:  "4.60" 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
  Current format:    QSurfaceFormat(version 4.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
     Version: 4.0
     Supports deprecated functions true 
     is OpenGL ES: false