Ok…
- Your initial question was:
- Yourself, you’ve decided to use filters to get a pure black & white result
- Your conclusion was that using filters to get this result is not possible
I gave you the solution with filters that is working: you have pure black & white result.
So now yes, use of filter is CPU consuming.
But it’s working as you’ve asked 
Now you’re talking about color space.
I think you don’t understand what is really a color space, and might confuse with color depth.
You’ll find here some explanations about difference.
If you want to work natively in pure black&white pixels, without any filter or too much CPU load, you have to work in a 1 bit depth color mode.
This mode doesn’t exist in Krita.
The nearest solution for this:
-
Use Gimp:
But, even Gimp is not really working in 1 bit depth mode; it’s a 8bit depth limited to 2 colors palette
-
If your goal is to work in black&white with a reduced memory footprint, the best you can have in Krita is the Greyscale/Alpha 8bit integer/channel mode (ie: 16bit per pixel instead of 32bit)
And then, if you don’t wan’t to have any greyscale, use pixel art brushes to work:
– 3 are provided by default with Krita
– More are provided by @Rakurri’s brush set:
Rakurri Brush Set - Free Krita brushes 😁 - #14 by Rakurri
Or just apply a filter to your picture at the end
Grum999