Execution gets very slow with increasing number of LEDs / Possible solution: Parameter "First LED"

[bodyhey]

I'm working with the Adafruit "Larson Scanner Shades" sketch - but on a larger scale with 1m / 144 LEDs. Even with the delay set to "0" the speed is not, what I need. Experimenting with different numbers of LEDs, the speed seems to increase over-proportionally. Is there the possibility to implement an "offset"-parameter for the first LED in use with the program ignoring (and not calculating) all LEDs before? With that, we would get the option to divide long strips into "substrips" in software without messing with the hardware. Or is sending the data the main factor, slowing down the whole thing?