5) Germans over-engineer stuff.
We're talking about bottom-loaders, not Contaxes or Contaflexes. The Leica is pretty crude compared to the absolute over-engineered nonsense the lads at Zeiss put together over the years!
I shot with a IIIf for a year some 20 years ago and my best loading time was 45 seconds while walking with the camera around my neck. I used the card method. The last time I tried to load a IIIf I couldn’t do it. The film kept getting caught and I ended up cutting the leader. I have a IIIG coming so practice practice practice.
If you're using a business card (or a card of any kind), you're doing it wrong. That's why it takes so long.
Cut the leader of your film before you leave the house (all my rolls of 35mm, bulk or commercially loaded, get the long leader - every camera can use it), and it's a pretty quick process. Unhook the baseplate, hold it in your mouth or pocket, remove the take-up spool and push the leader into the flat spring, slot it into the camera, quickly check the sprockets have engaged, replace the baseplate.
I just timed myself loading a few different cameras at my desk, with each time including getting the film out of the canister to begin.
Leica III: 26 seconds, start to finish.
Removable back Contaflex: 31 seconds.
Swing-back SLR, here represented by an Edixa-flex: 50 seconds. The film came unhooked from the take-up spool the second I tried winding on, and I had to do it again.
The biggest difference with every system for me is how well the take-up spool grabs the film. I'm sure everyone here has at some point in their lives had the utter nightmare of getting to "41 shots" on a 36 exposure roll because the film has unhooked from the take-up spool, and you didn't think to check the rewind crank after loading it. I've never had this on a bottom-loading Leica (but I have, in all fairness, had it on a Zorki 5 with a particularly knackered take-up spool), but it's happened on multiple SLRs a few times. I just don't trust 'em!