My journey was from film SLRs to DSLRs to mirrorless and gradually film alongside mirrorless with a mix of autofocus compacts, TLRs, SLRs and rangefinders. Manual focus takes both time and mental processing power that autofocus doesn't. Rangefinders are an antiquated technology from a bygone era and yet we have decades of techniques developed and passed down from many thousands of master and amateur shooters who used rangefinders for fast action capturing candid shots. There are two main tips for speeding up manually focusing with a rangefinder:
1. Zone focusing
This is where you use the distance scale on the lens barrel to set your focus in anticipation of how close you expect the action will be. You usually have to be shooting with a 50mm lens or wider and have the lens stopped down to at least ƒ/5.6. But do this technique enough and you basically get to use the camera as a point and shoot. It's faster than any autofocus.
2. Focus tab
A lot of lenses for the Leica M system have what's called a focus tab, which on Leicas is a semicirclular slot on the focus ring for your finger to fit in. This way, you can adjust focus with that one finger. Across most M lenses with a focus tab, the focus distance is consistent at different points, so the 6 o'clock position is one consistent distance, the 4 o'clock is another, etc. I forget what distances they are; neither of my Leica lenses have focus tabs. It's also common for Leica shooters, when they have an eye on action coming up, to move the focus tab as they're getting closer or farther to keep it floating on the subject. The fact that the viewfinder image doesn't get blurry when the lens is out of focus can help prevent you from being distracted while doing this.
Rangefinders are just cameras with one type of focusing mechanism. They're still a box that a strip of film or a digital sensor to goes in. Leica wasn't the only company to make rangefinders, but it's one of the only one who still does. They're not inherently better than SLRs but the rarity of rangefinders and the fact that one of the only companies that still makes them charges a premium for them has created a mystique around rangefinders. They have not been goods with mass appeal for decades now. Some people who try them just don't like them and that's fine. I have an M3 and the build quality is really nice, but the density and mechanical precision don't seem that much more magical to me than a really nice mechanical SLR. I enjoy shooting rangefinders, just as I enjoy shooting SLRs. As far as how long it takes to get used to shooting, that will depend on how much you practice with it. Rangefinders aren't for everybody but I think the particular challenges of shooting with them are overstated. It's fine to not like them but getting the hang of one isn't that hard. Like with anything it takes practice.