In general the longer the line the more power the kite generates at the turn, but it turns slower.....
Same way: shorter lines = faster turn, but less power at turning.
It is because of the change of the wind window: longer lines = larger wind window, more time for the kite to travel...etc...

It also depends on the design of the kite. I found putting a frisky small kite in gusty strong winds onto longer lines helps the control, and putting a slow big kite on shorter lines improves turning speed.

It also depends on the riding style. Wave riders prefer shorter lines for quick turns. Freestyle floaty jumps may need longer lines...etc

Usually putting a kite on longer lines increases its bottom end as well...

so if I'm starting with 23m lines out of the factory and am looking to improve the speed of my kite will taking it down to 20m be noticeable?
should I go 19 or 18m lines?