array-chunk | Chunk array into small arrays specific length | Base64 library

Answered here.

Your chunks/blocks are in two dimensions, so you can index them in two dimensions

I created this function to do this, but I guess it needs more improvements:

There's an important difference between the two options. In the slow version, you are concatenating each iteration onto the entire hex string you built up for each byte. String concatenation is a slow operation since it requires copying the entire string. As you string gets larger this copying takes longer and you copy the whole thing every byte.

In the faster version you are building each chunk up individually and only concatenating whole chunks with the output string rather than each individual bytes. This mean much fewer expensive concatenations. You are still using concatenation while building uyp the chunk, but because a chunk is much smaller than the whole output those concatenations are faster.

You could do much better though by using a string builder instead of string concatenation. StringBuilder is a class designed for efficiently building up strings incrementally. It avoids the full copy on every append that concatenation does. I expect that if you remake this to use StringBuilder both versions would perform about the same, and be faster than either version you already have.

You can do it a little easier

You could iterate your values and fill array using modulo :