I always thought it's not that the kite is quicker, the wind window is smaller. So from one side of the window to the other takes less time for the kite to travel, making it feel quicker. I might be wrong

I have always used 25m. I tried 20m for a while and didn't really like it, probably/definitely just not used to it though. I wanted to use my 8m in higher winds, so I would put on shorter lines. It didn't help all that much and I found that i was often very close to putting the kite in the drink - I just wasn't used to the smaller window. If you try your kites on 20m lines be prepared to sharpen your reaction time.
Kites on shorter lines look a lot bigger from your own POV when kiting. Whereas my 4m kite looks ridiculous on 25m lines