Moreso something I noticed when using some CPU intensive plugins, if I have 2 separate tracks (not bussed) each running a single instance of an amp sim (Neural DSP in this case), the CPU processing stacks onto one CPU core. This is when the tracks are not armed to record, but both have the same input channel from my interface selected.
It seems that any tracks which have the same input selected in S1 have all of their VST processing stack on one core, but when changing the input source to "None", multi-threading takes place and S1 splits CPU usage over mutltiple cores.
I don't know if this is by design, or an issue with Studio one, but I've seen a lot of threads of people struggling with high CPU usage, and this is one thing that helped me out.