If you split the code horizontally instead of vertically you have to deal with IPC, which could be problematic (and slow). I would do that only if there was alr

If you split the code horizontally instead of vertically you have to deal with IPC, which could be problematic (and slow). I would do that only if there was already a clearly defined interface between the modules, not just for the sake of concurrency. Moreover, taking into account growing number of cores in modern machines such strategy doesn't seem very future-proof.