it could be expressed by the distance a particle travels in 'Short = 30 minutes', 'Long = 5 hours'.

With the current settings, it’s the opposite: a short vector means more time e.g 5 hours, a long vector less time e.g. 30 minutes, to cover the same distance.
As @Gkikas-LGPZ explains, the best solution to know the running time of a particle on a given distance is to use the Distance&planning tool.

and with observed wind speed e.g. 10 kts

you can see that an air particle over Wembley will be after 1 hour
just north from Enfield.

So if there's barely any wind, setting it to display the length it travels in 5 hours would still be a short line, which is exactly the same as now, but would give better intuition for me to understand the strength of the wind with real life measures (it could go like "oh, in 5 hours a particle in this location is about to move from Oxford to London, takes less time to drive there by car, so the wind is pretty weak").

What do you think?

In addition, using my customized wind color scale, the Wind layer is more intuitive (at least for me).

If you want to have the wind vectors shown at « real speed », consider a strong wind at 60kt i.e. more than 110km/h. Along a Great Circle, the wind vector should move with a speed of 1° per hour. Just imagine what it gives on your screen...approximately no movement !
Use for that the Distance&planning tool.

To see a particle move over this distance, you would have to stay one hour in front of your screen.

I'm sure you realise that Windy's particle animation moves more slowly in areas of low speed relative to windier areas. The "tail" of the particle is also longer where it's windier. If I read this right, you want the wind to move at the pace of the "real" wind. But you can't – you wouldn't see it move.

Imagine a frontal system postitioned over the Azores in the mid-Atlantic, which is moving eastwards with a speed of 15kt. If you were in the Azores, you might experience a wind of 15kt which is noticable. But by the time the frontal system has reached the Bay of Biscay 2000km away, 3 days will have passed. So basically, if Windy visualised a particle within this airflow, the particle would take 3 days to move across the screen.

The particle animation therefore appears to have winds that flow THROUGH features like weather fronts, which makes no meteorological sense... but it gives a great "feel" for where the stronger winds are and how they curve on a planetary scale.

Does that make any sense? I might be way off base, please feel free to clarify your question :)