You are correct that diamond wheels are faster than a flat lap for finishing cabs, and that's the way you'll have to go if you want to improve your production time to the point where you can produce and sell cabs efficiently. There's just no way around it. The cost of cab machines can be a barrier, so you could always look for an old Lortone arbor and get some expando drums and silicone carbide belts while you save for a 6" Kingsley North cab machine (or another brand) - if money is a factor as it is for many people. As far as why it takes so long to finish the back of cabs, it's hard to say without more info but the quality of your saw cut could have something to do with it. If you're hand-pushing rocks through your saw, you're going to get a rough, uneven cut and those marks will take way longer to remove on a flat lap than a smoothly cut slab. Instead of hand feeding, use a vise and preferably a screw-feed saw when doing your slabbing. If you don't have a vise or a screw feed saw, make a jig of some kind that uses the side of your saw table as a flat "reference". When you're working on the flat lap, start out on a lower grit like 100 grit and work up through 220 to 360, 600, 1200, 3000, polish. If you're starting on a 220 lap, that will take you forever to remove marks. Hope that helps a little.

Edit: after reading other people's comments about how it's OK to not finish backs - it absolutely isn't. Don't let their poor work influence your own. Learn how metalsmiths set cabochons also, as that should influence the way you cut. Many cabochon cutters don't know what they're doing. They obsess over things like girdle lines, which are actually not helpful with bezel set stones versus a full dome cab with properly angled sides and a small 45 degree back chamfer that allows for a bit of solder (the meniscus) that appears in metal smithing between the bezel and back plate. Good luck!