I'm not entirely sure, but it must be more expensive to concatenate with the longer string on the end. Even the one that passes takes about 6 seconds. The docs mention that there are often performance costs associated with working with strings and that it may be adventageous to work directly with binaries. Let me know if you're able to optomize it (I may take a crack at it later). Maybe a line could be added to the description warning of the high cost of String operations in Elixir.

Cool. Thanks... I was wondering how cold this be done in a nicer way.