Cut the string into chunks (a chunk here is a substring of the initial string) of size sz

sz is the size of each chunk, not in how many chunks you should cut the string. I don't see what's confusing there.

The tests contain some very huge arrays, so think about performance.

That's already in the kata's description.

What makes you think that 9 lines of code has anything to do with how fast the code is? Apparently, your 9 lines is too slow. You need a solution which is fast, even if it has 50 lines.

There are two similar tests, one that's fully uppercase and one that's not. The first one expects true, the other, false.

It's not the kata's author who sets the kata's rank.