FDTD will try to use all the available memory bandwidth even if it will not use all of the memory or not use all CPU cores. When all of the system's bandwidth is used, it will slow down other running processes on the machine.
A single user can run 2 or more parameter sweeps (concurrent parametric computing) on a single machine - for simple/small sweeps than runs on a few cores and taking only a few MB of memory.
It is not recommended to have more than 1 user run FDTD simulations on the same machine. If 2 users run FDTD, both simulations will slow down significantly depending on the simulation's components/design/requirements.