I have a micro 4/3 camera (Panasonic GX9) with MF assist. I would like to experiment more with vintage and vintage-remake lenses without AF, and thus I need to learn to manual focus.
I can do it when the subject is still, but it takes a few seconds. My current lenses are focus-by-wire, but they are very responsive so I don’t think that is an issue.
But generally, for moving subjects (eg an animal that is moving around, or flower that is being moved by the wind), my focusing skills are not rapid enough for practical purposes.
Is this something one can learn? What would you suggest? Or should I accept this as a limitation of my camera and stick to auto focus? Or just accept that I can do MF, but maybe only 1/10 of my photos will be in focus on a good day?
Suppose your object is walking/running/cycling/driving towards or away from you. Focus on a spot on the ground you think they will pass. Fractionally before they pass the spot, press the shutter.
I would say yes but - with some limitations. Only suggestion is lots of practice! (not very helpful I know)
I got a lot of practice back in… maybe 2012?.. my camera was a Pentax K-01 (THE Pentax APS-C mirrorless ) and the contrast detect AF was very slow especially with my screw drive Sigma 70-300. But I wanted to shoot birds so I just kept trying and worked the best way to hold the camera, which way to pull focus so as have the best chance of catching it, a custom button for LV magnification, and so on. I don’t really have any specific tips though. I got quite good at it.
Thanks to all of you for the advice and encouragement. Then I will just have to practice.
From looking at the math I get the impression that focus degrades more quickly in front of the the focus plane (closer to the camera) than behind the focus plane (farther from the camera), the extreme case being hyperfocal focus at h with the focus plane extending to h/2.
So, allowing for error, does it make sense to focus a bit closer than the subject? The idea is that if it moves a bit closer, it will be in focus, and if it moves a bit farther, it does not blur much.
I don’t know about the theory - but FWIW in practice I almost always prefer the look of front-focus if slightly off. Looks more like it was intentional, whereas (to me) focusing slightly to far just looks wrong.
Don’t forget that the average amateur shot was not really sharp by today’s standards. I’d risk saying that most professional shots were not that sharp, either. A random sample (it’s low-res, but the text is much sharper than the photo):
Yes, could be a matter of taste. But I still think lenses have improved so much that most 50±year-old pro photos would no longer be accepted by today’s stock photo catalogues, magazines or even web publishers.
Following Roger Cicala’s advice to take photos of grass to explore field curvature, I use Gimp’s Edge Detect / Image Gradient filter for practical exploration of DOF on my lenses at various apertures and focus distances (image below f2.5, white band in middle shows the grass that happens to be in focus).
(I wonder if there is something similar in Darktable so I would not have to open Gimp in addition?)
) and the contrast detect AF was very slow especially with my screw drive Sigma 70-300. But I wanted to shoot birds so I just kept trying and worked the best way to hold the camera, which way to pull focus so as have the best chance of catching it, a custom button for LV magnification, and so on. I don’t really have any specific tips though. I got quite good at it. 
