If it times out it is either inefficient, either it gets lost in an infinite loop in some cases. Nobody can know, you don't provide any information. Please refer to the documentation: https://docs.codewars.com/training/troubleshooting/

The individual test run from around 20ms or 2ms but it still timed out ? Is my algorithm still inefficient ?

maybe you are printing somthing

My algorithm does a 20k array in 0.45 seconds. Why is it still slow?

.

Is multithreading the solution ?

From the Kata description.

rotors // array of array-of-rotor-values. Each array-of-rotor-values is applied to the corresponding display line

Why is the rotors 2D array and not just a simple array ?

Not a suggestion.

My first attempt failed because there's a System.out.println() inside the for loop. After I removed that it passed all tests.