Speed isn't actually too much of an issue with the ruby client, since all the CPU time is spent in the GnuTLS and Digest::SHA256 code, which is already written in C.

The problem is portability to android and iOS. We additionally want to make the core easy to embed in other applications.

He mentions an android client. If you want it to be fast, or not drain your battery, you need Java or C++. If you want it to run on iOS or Windows Phone, C++ it is.

To make it easier to port to Android and IOS is my guess.