WHAT IS IT
Fast Acceleration is a technique in all current versions of Venineth, where you can make the marble accelerate its roll faster than intended, at the cost of requiring more skill to control. It is typically done by using multiple movement input methods at the same time.
Attemping to ban fast accel by restricting people to only using one input method at a time is both futile (as you can do it with only one input method) and leads to unplayable runs (optimal movement is staring at the floor, rolling sideways, memorizing what is ahead of the camera).
For this reason, the Venineth leaderboards should permit the use of input remappers to allow for more convenient use of fast accel.
HOW DOES IT WORK
Venineth has four input methods for movement: controller analog stick, controller d-pad, keyboard WASD, and keyboard arrow keys. If you press forward on an input method, you will spin forward with one unit of acceleration. If you press forward on 2 (or 3 or 4) input methods, you will spin with 2x (or 3x or 4x) acceleration. This is FAST ACCEL: going faster than 1x acceleration.
It is important to note that depending on the surface, if your marble is accelerating too quickly, not all of its spin will transfer into roll. According to my tests, the highest friction surfaces will only accept up to 2.25x accel, and some accept even less than 1x.
HOW CAN YOU USE IT IN A RUN
If you have a custom controller or very dextrous and healthy hands, you can claw the left side of your controller to use the analog stick and d-pad at the same time. If you have hand pains, however, this is untenable for a 2+ hour speedrun.
My alternative proposal is to use input remappers like JoyToKey, to allow keyboards and controllers to simultaneously send multiple controller inputs and/or keypresses. By the strictest definition this is a kind of macro, but it is a very weak kind that does permit any inhuman inputs or TAS-only behaviours, which is the spirit of the ban on macros.
In practice, you can set up your controller to use fast accel inputs by default, and revert to normal inputs when e.g. a button or trigger is held down. You can also control how many inputs are used in each direction, e.g. 3x when going forward, because you want to accelerate, but only 2x for backward, because braking strongly is more likely to make the marble slip.
SHOULD IT BE BANNED
If you press left or right on a 4-way input (d-pad, wasd, arrow keys) you will roll left or right with 1x acceleration. This occurs independently of forward or backward, so diagonally you will roll with 1.41x accel. If you press left or right on the controller analog stick, you will roll left or right with 1.83x accel. So it is effectively impossible to force people to never go above 1x accel.
If you force people to only ever use one input method at a time, then the optimal strategy would be to roll sideways, on controller. Due to the pursuit behaviour of the camera, this will make it impossible to see around you, so you would have to memorize the layout of the levels (which makes it very difficult to play) and orient the camera at the ground for maximum information (which makes it very difficult to spectate).
Allowing fast accel but banning input remappers would unfairly advantage those who have access to special or even custom controller hardware that makes fast accel convenient to input.
Allowing fast accel and permitting input remappers (while still banning all other kinds of macros) would permit any control scheme (contoller, keyboard-and-mouse, etc) to have the same maximum potential, as well as complete the game as fast as its mechanics allow.
