Which version is having this issue when running concurrent FDTD simulation as compared to which older version as per your post;
In theory, the simulation time should only double since each simulation occupies 50% CPU, and was the case previously.
FDTD performance and utilization is not linear. FDTD Is very CPU and memory intensive. It will try to take up all of the available memeory/system bandwidth on the machine. It may be only running with 50% of the CPU cores but it will be running with all avialable memory bandwidth on the machine.
>>> Information on Hardware Specifications – Ansys Optics
Sometimes, depending on the simulation/sweeps, it would be faster to run them in sequence than running multiple simulations concurrently. We have this KB guide below that might come handy.
>>>Determining the Optimum Resource Configuration to Run FDTD Simulations – Ansys Optics