With a cartridge system, it is damn near impossible to cut yourself even if you tried... With a safety razor if you hold it at the wrong angle you'll get cut (and the angle depends where on your face you're shaving, so it is very much a per-face learnt skill).
The main reason why a safety razor takes so much longer is that you need a good lather from the shaving soap. This offers the most lubrication, which will protect your face from the sharp blade. The crappy supermarket shaving foam just won't cut it (minor pun intended).
I use the non-foaming gels with a cartridge, which is fast and smooth. I gave a safety razor a real shot, but I don't want to spend 20-30 minutes before working shaving, I want to spend 5-10 minutes, and have almost no risk cutting myself no matter how reckless I am.
But undeniably safety razors are sharp and cheap. They're just a lot of hassle.
That said, you don't really need the adjust-ability to deal with beard thickness, you set the blade gap to match your skin, the angle you shave at, and your blade selection. The idea is more gap means the razor cuts deeper into the exposed hair, giving a closer longer lasting shave. If you get too greedy, you eventually cut your face. Sharper blades typically need less blade gap than less aggressive blades. Adjustable razors just make it easier to tune for the other two variables you can control (sadly it does nothing to make your skin more robust ;).
Having a single disassemble-able razor is what helps with beard thickness. The hair just washes away rather than getting stuck in between the blades. Cut beard hair is fairly sharp and irritates the skin as it passes over your face. It also bends the blades and changes the angle the blades travel at. That can also increase irritation.
For a large swath of the world that isn't a big deal, but it makes it darn near impossible for me to get a good shave.
When I travel I use a non-adjustable 53 Gillette Super Speed[2] due to its light weight and size. It's not nearly as nice as the Futur but responds well to the Astra blades for a shave that is as good as I would get with a Mach III but without the cleaning needs and less irritation. I bought it for $20 off of ebay and it's not quite as pretty as the linked one.
[1] http://www.amazon.com/Merkur-Futur-Safety-Razor-Finish/dp/B0...
[2] http://www.razoremporium.com/gillette-1953-notched-super-spe...
Brain dump:
http://www.shaving101.com/index.php/education/double-edge-sh...
http://www.shaving101.com/index.php/education/double-edge-sh...
http://www.shaving101.com/index.php/shaving-faqs/253-do-i-ne...
Part of the time savings is going with a tubes of shaving cream. The cream (Proraso in my case) lathers quickly and consistently, and is vastly superior to the canned stuff.
In the end, after becoming fully adjusted, I find that safety razors are just as convenient, give a better shave, and are far cheaper than cartridge razors. It takes just a minute or two longer.
With those shaving creams, I find that it pretty much doesn't matter what brand of blade I use any more, and I can shave in 5 minutes. If you really wanted to optimize on the margins, you could get a sampler pack of 10 different kinds of blades and find the one that works best for your skin chemistry.
Lathering the Proraso takes seconds, rinse my brush with really hot water and put some hot water in the Proraso tub for a few seconds before dumping it out. Swirl around the brush for a few seconds and it's good to go.
For what it's worth, Proraso works well with cartridge and disposable razors also.
I typically wait until my facial hair is pretty thick before shaving too. I use Jack Black's Face buff in the shower, then Jack Black's Beard Lube and layer Proraso on top of it. I'd be interested how well my shave would be if I used the same method with a cartridge.
I just wait until I'm done washing and the steam has had a chance to soften the hair, then shave directly in the shower. It took a couple of weeks of shaving daily for my skin to get used to it but now I get it done with no irritation. An advantage of no lather/gel is that I can keep putting my face under the shower to keep the hair soft.
(I shave without a mirror, and then clean up afterwards)
This isn't going to be for everyone, and it certainly isn't as close as a gilette, but I can get the shave done in a couple of minutes.
It takes a tad longer than a three blade razor, but I've always had closer and better shaves.
I've tried disposables, cartridges, electric and the safety razor and I'd rate safety razors better than all of them.
However, I still use a my Fusion daily, since it's so fast and efficient. With a safety razor it takes time to lather up and get a proper shave.