First you need to determine why the drainfield is slow. If the problem is high groundwater, nothing short of elevating the field or lowering the water table will help.
Around here, fields that once worked but then clogged up over time simply get a new elevated field installed. IMHO, they look terrible having a giant mound in your yard.
The codes changed here. At one time the field could be flush with the surrounding grade but now one can only go down 2 feet and so need to go up 3 feet.