Bug 421098

SUMMARY
This is the case for all versions.

The Speed transfer curve has input values from Slow (zero) to Fast.
The output is 0% to 100% of the controlled parameter/property.
(As far as I can tell, the Speed is proportional to 'canvases per second'. This results in the behaviour being independent of different canvas sizes but a given manual stylus stroke has a higher determined speed at higher zoom levels.)

The Fast speed is so large that even with totally uncontrolled 'swipes' across the canvas it's very difficult to get near to the 100% output value.

STEPS TO REPRODUCE
A detailed explanation with illustrations has been presented at:
https://krita-artists.org/t/does-anyone-use-speed-control-in-their-brush-presets-a-proposal/6181

PROPOSAL
Any change to the current behaviour must not affect any existing brush presets that use Speed.

The following is proposed:
The Speed transfer curve control should have a 'scaling' slider added that shows a scaling factor of 1 to 100.
The 100 setting should be the default setting and correspond to the current behaviour.

If the scaling is set to e.g. 50, then the default/current value of 'Fast' would be reduced to 50% of that value. (Or the determined input speed would be multiplied by 2 before being passed as input to the current transfer curve.)

ADDITIONAL INFORMATION
Simple (and limited) experience shows that a scaling factor of 10 would give access to the full transfer range at 'reasonable and practical' speeds. Anyone who wanted to use fast swiping actions could set it to 30 or higher. Someone who wanted to use relatively slow speeds to fully control a brush property could set it around 5 or lower.