SystemModeler simulations may initially run slowly and then speed up. This is often caused by overspecification of the initial conditions in the system. Overspecification happens when there are too many initial equations. SystemModeler uses an overdetermined system solver to find the values of overdetermined systems that can result in slower speeds.
Models are often overdetermined if they contain too many fixed variables. A variable may be fixed by setting
fixed=true in the Modelica code or if the variable is set to fixed from the user interface like the following example: