I can boil ~7.5 gallons in at most 40 minutes (seems more like 30 but it's hard to tell cuz I don't go from tap to boiling right away), though I did purchase a canning element to replace the large coil on my stove (it outputs more heat than the standard element). That was about $20 or so. I place the pot on both the large coil and half the back coil so I can use both.
The key is good insulation. I thought about a jacket or something for the pot but decided that'd be too difficult for me to pull off, plus most of the heat is lost either going out from the sides underneath the pot or into the metal of the stove itself. My cheap strategy is to remove both element, place a 'vertical' layer of foil down into both holes, then place 'horizontal' layers of foil across each one. I put the elements back into place by punching holes in the foil and inserting them like normal, then the pot goes on top and the excess foil gets turned up, basically creating a "bowl" of foil that that pot and coils are sitting inside of.
I just take a couple more large strips of foil and wrap them around the pot maybe with an extra layer across the bottom. This way, sooooo much heat that is normally lost is transferred into your wort instead!
I save as much of the foil as I can and reuse it, but eventually it winds up tearing and I have to use new foil. The foil that goes underneath the coils always gets ruined though.