Sclang runs on a single thread. If you want to multithread, you can do the calculations using UGens and use SuperNova. Or you can fork entirely separate processes using .unixcmd. That could be another instance of sclang or it could be C++, python or whatever you like.

You can manage I/O with temp files, OSC messaging, or returning data using the action function for .unixcmd.

See https://scsynth.org/t/parallel-processing-in-sclang/12509/2 more more information.

Note that if you want to distribute your code, you can't rely on any particular interpreter being installed on their system - not everyone has python or even java. Compiled code is generally not cross platform. They will have sclang, but starting a new instance of the interpreter involves compiling the entire class library and has relatively high overhead.