A brush I'm creating runs too slow. The tip is 300x300 pixels (can't go lower) and the spacing is 0.05 in Auto mode (can't go higher).
The problem is I think the tip I created stores a useless alpha channel (it's just greyscale), so I enabled "Use color as a mask" but I wonder if I could avoid storing the alpha channel when creating the tip because the default brush tips don't even need to have "use color as a mask" enabled, while they have a white background.
Or perhaps you have any other tips to make brushes faster ?

I'm not surprised it slow, 0.05 spacing in Auto mode is very, very low spacing and require a ton of time to compute.

If you show how this brush should looks like I can give you a better tips and maybe make the brush?

Well I did the job with the slow brush, but my question is not especially for this particular case, I was mostly curious about this alpha thing.

Honestly I don't know if alpha affect brush "speed" but I always check "Use color as a mask" at creation of the brush tip.
Most important performance setting is the spacing, try ti use the highest possible value (that looks alright). And don't use auto spacing, it tends to decrease spacing (slower brush) for bigger brush sizes.
Also, Precision:1 can speed up a bit. I personally don't see the difference between precision 1 and precision 5.