No, Reaktor does't use multiple threads (It's actually technically very difficult to implement multithreading in a modular emulation without adding delays to each connection). But if you run multiple instances of Reaktor in your DAW then the DAW will spread the load across cores.Just a random question: do polyphonic blocks use multiple threads / cores, or run as a single thread? Last time I checked, Blocks ensembles were heavy on CPU.
The polyphonic blocks can be quite taxing on CPU though. This is due to the quality of the filters and oscillators and the fact that the whole modular rack is running always at audio rates with no latency, so that the feedback possibilities of an analog modular rig can be properly emulated.
So best to use the monophonic versions of the blocks unless less polyphony is really needed.