I don't believe that modelsim is capable of multi-threading the simulation kernel. I don't think any simulator does that. Modelsim will put the update of the waveform display on a separate thread, but that has nothing to do with the simulation kernel.
Most of the performance improvements due to the machine will come from using a computer with the highest clock frequency processor with the fastest RAM and an SSD drive. If you want significant performance improvements in the simulation time and don't mind losing visibility into the design then using vopt options will benefit you the most (even more than an overclocked screaming fast computer). Of course that means using the higher end versions of Modelsim/Questa as the low end ones don't have any optimization capability.