Leaf shutter operation uses a timer delay mechanism for speeds from 1 second to 1/125 second. There are two adjustments on the delay timer, the travel of the operating lever and the tension on the pallet. Slow speeds use the maximum travel of the operating lever and the maximum tension on the pallet. The main cocking spring supplies the power for the timer to run, the speed dial the travel and tension settings.
On leaf shutters that run slow the travel lever and pallet tension move but the gearing turns slowly and the pallet does not move easily resulting in slow speeds.
On shutter that run fast or some/all speeds the same the travel lever does not move the full distance of the space allocated to the set speed. Sometimes it moves a little other times it does not move at all.
Send the camera out for a CLA (clean, lube, adjust).