Shorter lines dont make the kite faster.
But of course it reaches the edge of the window faster - VERY important difference.
19m lines i fine for most conditons I think, for a small kite.
I dont like short lines myself - as the kite tends to be way too sensitive in waves, so you dont get the same consistent pull but an on/off very uncomfortable ride
You can always depower a good wavekite (not if a C kite though...), so no problem there - but you can never get the consistent pull you want when going out f.ex, with short lines - as the wind are typically sideshore and gusty, besides lee and turbulence from big waves in the impact zone.
So I just use 20-22m lines for all my sizes nowadays, in waves - and only for super light wind I might use 30m but not waves of course - but I dont like shorter lines for the 5 or 6 or 7m2, as the wind is often even more gusty when blowing.
Others DO like 18m lines in waves, so very personal.
And also kite specific - some kites can not depower as much, so a lot of the mainstream SLE kites need shorter lines when riding waves in high winds, as they would otherwise pull to much and too long when flown deep and across the window going down the line
Other kites can depower sufficient - so you can use normal (and more efficient/more range) lines.
The Evo can not depower much, so shorter lines needed in high winds, to be "comfortable" in waves.
The Neo is more a real wavekite, and depowers good, so you can use more standard like lines with the advantages it gives you because it will smooth out the wind gusts so you can focus on the waveriding and not the pull
Because it is so kite specific - and depending on conditions and what you do and how you ride - I think we see so many opposite opinions about "short lines" in a forum like this
Also - it takes a lot of time "getting used to" - as short lines has this on/off feeling, that some can adapt to, others dont want it.
For flat water I have no idea whatsoever - as I dont have flat water when normal winds (small kites), and only ride flat water extremely rare (in 10 knots with a big 12m2 kite)