I’m currently in the train and will be away from my workstation for a week… use of my laptop is not possible for such a test 
But I’ll try to test after vacations.
Here some informations:
If you have some technical knowledge about C/C++ programming, the most important information is at the end of the post, concerning how the C free() function manage released memory…
Made my test on Krita 5 pre-alpha, but I’m not sure that about this point there’s so much differences.
Usually, brush size is limited by default to 1000px because greater values might be unusable in terms of performance. Not for all brush engines, but some brush engine are very complex.
Probably the RGBA brush engine is not the most simple 
Don’t remember if @Voronwe13 or @dkazakov implemented the brush engine and can gave you better answer, but I’m not sure if it’s possible to improve RGBA brushes speed/memory impact… 
Also note that I’m not sure developers have workstation like yours (or even like mine) and that could be difficult for them to test software in such conditions
Grum999