Hm, my formula is really simple and works at least for industry and blue robots:
Speed = (Base Mass/Actual Mass) * Base Speed * (1 + Bonus from Navigation)

Base Mass is the mass of a robot with no added modules. (Ex.: Sequer = 10500)
Actual Mass is the mass of the robot with added modules.
Base Speed is the speed of a robot with no added modules. (Ex.: Sequer = 43.2)
Bonus from Navigation as float. (Ex.: 18% Speed Bonus -> insert 0.18)
Greetings,
Holok

PS: Lightweight frames only reduce the robots frame mass. So with a LWF the actual mass is base mass * (1 - LWF reduce) + weight of modules.