I don't know anything about the Hass door, so this might be no help at all...
I would first disconnect the air supply and then manually slide the door back and forth. This will check there is nothing binding. It will also tell if the cylinder end-of-stroke cushions are working.
If the load (door) becomes stiff at some point, an air cylinder will build up pressure to overcome the load, then move faster as the load is overcome. If the stiffness was just before the end of the stroke, slamming might occur.
Two of the suggestions made in previous posts need to be considered separately. Cushion and speed are separate issues.
Air cylinders usually have an adjustable cushion built-in to each end. It allows the piston to stop gently at the end of its stroke. The cushioning is adjusted by a needle valve (usually with an allen key or screw driver).
However, if the door is fully open (or shut) before the piston reaches its cushion zone, the door will slam. (The cushion is only available at the very end of the piston stroke). This would occur if the piston stroke was greater than the door stroke.
(I have seen separate mechanical shock absorbers used to provide cushion to a heavy load when cylinder cushion was not available, but I doubt it applies here).
The speed which the cylinder moves is a separate issue. A common method of speed control is to restrict the exhaust from the "other" side of the double-acting cylinder. This ensures the working side of the piston receives full air pressure, but the exhaust air from the other side of the piston is restricted, thus slowing the piston speed. The exhaust restriction is often an adjustable needle valve combined with an exhaust silencer. It may be found on the cylinder port or the solenoid control valve exhaust port.
So cylinder cushion and flow (speed) control are separate issues, but commonly both are needed.
This was the first YouTube that popped up when searching. Note that the adjusters fitted to the cylinder ports have no effect on the air entering the cylinder, just the exhaust air coming out. I prefer to fit them to the solenoid exhaust ports so they can be adjusted without getting near the cylinder while it is working (but some solenoid banks are probably too miniaturised to allow this).