Created attachment 139282 [details]
example file ready to use

SUMMARY

Flatten layer action is very slow when applied to a paint layer on which an animated transform mask is attached


STEPS TO REPRODUCE
1. Create a document with a paint layer
2. Add a transform mask to paint layer
3. Define a 600frames animation with animation curves applied to transform mask
4. Flatten paint layer

(or download attached example file)


OBSERVED RESULT

With a 24CPU/64GB RAM computer (with 12CPU for Frame rendering clones limits)
- Render animation as PNG sequence: took around 15s 
- Merge paint layer with transform mask: took around 1hour


EXPECTED RESULT

It shouldn't be so long to flatten layer.
If rendering 600frames as PNG files took only 15s, we can expect flatten should be as fast as rendering (or even if flattening is a little bit longer than rendering for miscellaneous reason, it should not be so long)


SOFTWARE/OS VERSIONS
krita-5.0.0-prealpha-c25d2e1-x86_64.appimage