Blast furnace slag (BFS) is majorly composed of acidic and basic oxides and shows a complex, multiphase structure, which is expected to promote promising functionality for BFS for semi-crystalline thermoplastics such as PP. In the current study, the influence of variation of two internal mixing process (IM) parameters, mixing speed and duration, upon the rheological, thermal and mechanical properties of a PP-BFS compound is investigated. The rotors’ torque is observed to be directly proportional to the mixing speed, while the amount of consumed specific energy (E) during the mixing process is witnessed to be a function of the combined mixing speed and duration. Rheological findings showed that the complex shear viscosity and -moduli were modestly affected by the variation of the processing parameters. Thermal analyses revealed a modest influence of the processing parameters upon the degree of crystallinity (DOC) as well as the crystallization and melting temperatures, Tc and Tm. The neat PP recorded the lowest DOC and E levels, whereas the setting with 120 rpm screw speed and 20 min mixing duration (‘120,20’) recorded the highest ones. For the mechanical properties, the tensile modulus of the filled PP compounds showed a clear dependence on the variation of process parameters. The yield strength decreased notably as the process parameters progressed beyond the ’60,10’ condition. The ultimate strength levels were comparable for all filled conditions, while the highest ductility was achieved with the ‘60, 20’ and ‘90,10’ conditions.