This. I was actually looking for the time performance of other solutions compared to mine, but the only solution I had found was actually implementing all of them and time them myself.

I would definitely like a feature that shows the processing time of the code (or cycles),
I would love to improve my coding skills with focus on such "best practice".

'nt' ?

Agreed. On all test cases for Java "expected" and "but was" should be reversed.

Now we can sort solutions by "Best Practice" and "Clever"; both user tags. What about possibiliy to sort by some automatic metrics as well such as code length, complexity or runtime ?

In the linked wikipedia article it is mentioned: "When a computation is finished, its result remains as the top (and only) value in the stack". So basically I would assume an expression that ends with more than one value on the stack would be invalid and should raise en exception instead of returning anything.

Maybe the descriptions should be a bit more specific about the difference - since I was not really sure I just copied my solution from the other problem and it worked here too. For example something like "This is the exact same problem as PG-13 but it has to manage a matrix of size N in under M seconds" ?

Agree, this was my first 4kyo kata and it felt simpler than other 5kyo that I solved earlier.

In java all the assertEquals calls should swap the args. The expected value should be first.