Insulating your arch would save fuel, but for speed you either need to increase pan area or increase burn temps. Without a lot of work or money pan size area is the way to go, longer arch, more pans and only have the fire at the front with a limited amount of depth below the back pans so the smoke/heat has to rise and contact the pan surface instead of sneaking by to the chimney. I would avoid a water heater for boiling for toxicity sake. Saving time however can mean just not being there when it is boiling. On my setup I use deep pans and a large firebox. I am rarely near my boil more than 15 minutes per loading. I fill my pans, fill my firebox and leave to repeat either hours later or the next day. This does not give me a good gallons per hour boiling rate overall but my rate for boiling per hour I am there is decent. My ideal is at least two fires per day, the first heats everything up and removes some liquid, the second while everything is hot removes much more in the pans overall. My rig is remote to my house so this works best if I am doing some other chores nearby like cutting firewood. Fire, cut wood for a couple of hours, repeat. On a day where I can fire four times I boil off quite a bit and only spend about an hour doing it. My pans are homemade stainless steel, about 8 inches deep and the stove has a firebox that will hold a small wheelbarrow of wood.