Let's distinguish the valve under the front-center of the seat, which on the smaller Yanmars can have a T-handle, from the smaller T-handle 'drop rate valve' under the left or right side of the seat.
The one in front is to block the return line from the 3-point, in order to lock the 3-point in the up position for maintenance on an attached implement. This front valve's normal operating position is wide open, full counterclockwise on its knob or T-handle.
The smaller T-handle valve on the side of the transmission is the drop rate valve. Closing it partly (clockwise) will slow the descent of a heavy implement.
I don't know of a way to moderate the 3-point's lift rate aside from a gentle touch on its lever. But I have noticed my present YM186D lifts more slowly than the YM186D I sold so there must be some difference it how it is set up. Possibly this second tractor's power steering proportioning valve has an effect even when not steering, or possibly the earlier YM186D had the optional larger hydraulic pump.