[Thodge]

The easiest way to think of it is that the throttle shaft and lever are effectively a crankshaft and the linkage rod is like a connecting rod. Like you say with a crankshaft spinning a constant speed, the pistons move fastest at 90 and 270. The only difference is that the work is being applied in the opposite direction so 90 and 270 above and below the throttle shaft would give the slowest opening of the butterfly.
The tricky thing in changing the overall ratio of the linkage would be keeping enough travel to fully open the throttle. You would need to tweak the lengths of every lever in the system together or you’d end up with a very high pedal off the floor.