After re reading your post I see you have slow down issue..
Make sure the cable/valve is moving the full stroke thus opening the valve all the way .
There may be also be an adjustment screw to regulate the drop speed.. first I would make sure the cable is opening the valve all the way open.
It should have a cover on where the cable ties into the valve,take it off and stand behind the blade and have someone push the down position and see if the valve is moving all the way.If you see a full stroke then I would suspect out of adjustment or possible freezing of the down valve.
When did this issue start ? did it gradually start down slow going or do it all of a sudden?