I have noticed that SUMO's marouter uses multi-threading (--routing-threads XX) when re-routing or when using marouter to define the vehicle's routes macroscopically. This works when the routes are searched for incrementally (--assignment-method incremental), not however, when running UE or SUE. Any ideas why not? I am running SUMO 1.1.0 on Windows.
For anyone else wondering about the same issue. According to the developers, this is a current bug. See here for updates: https://github.com/eclipse/sumo/issues/5370